Add plan 89: TOC generator directive and MDS035 auto-fix

Follow-up to plan 88. Adds a <?toc?>...<?/toc?> generated-section
directive (MDS036) that emits a nested list of the document's
headings, and upgrades MDS035 from detection-only to auto-fix by
rewriting the four detected renderer-specific TOC tokens into
<?toc?> blocks that MDS036 then populates on the next fix pass.

Design notes:

- New rule MDS036 "toc" (meta, default-enabled, fixable) built on
  the shared internal/archetype/gensection engine that MDS019
  catalog already uses.
- Parameters: min-level (default 2, matches Python-Markdown [TOC])
  and max-level (default 6). Output is GitHub-slug links.
- MDS035 Fix leaves [TOC] untouched when a matching link reference
  definition makes it resolve to a real link.
- Relies on mdsmith's existing multi-pass fix; falls back to
  emitting populated content directly from MDS035 only if that
  proves brittle.

Author: claude

PLAN.md

@@ -46,4 +46,5 @@ footer: |
 | 86  | 🔲     | [Markdown flavor validation](plan/86_markdown-flavor-validation.md)                                             |
 | 87  | 🔲     | [Flavor validation for GitHub Alerts](plan/87_markdown-flavor-github-alerts.md)                                 |
 | 88  | 🔲     | [TOC directive migration aid](plan/88_toc-directive-migration.md)                                               |
+| 89  | 🔲     | [TOC generator directive and MDS035 auto-fix](plan/89_toc-generator-directive.md)                               |
 <?/catalog?>

plan/89_toc-generator-directive.md

@@ -0,0 +1,260 @@
+---
+id: 89
+title: TOC generator directive and MDS035 auto-fix
+status: "🔲"
+summary: >-
+  Add a `<?toc?>...<?/toc?>` generated-section
+  directive that emits a nested list of the
+  current document's headings linked to their
+  anchors (MDS036). Upgrade MDS035 (plan 88) to
+  auto-fix each detected renderer-specific TOC
+  token by replacing it with a `<?toc?>` block,
+  which the directive then regenerates on
+  `mdsmith fix`.
+---
+# TOC generator directive and MDS035 auto-fix
+
+## Goal
+
+Give mdsmith a native heading-level TOC
+generator so MDS035's detection can be followed
+by an actual fix. Today MDS035 only flags
+`[TOC]`, `[[_TOC_]]`, `[[toc]]`, `${toc}` and
+points at `<?catalog?>`, which is a file-index
+directive, not a heading-outline replacement.
+
+## Context
+
+Plan 88 landed MDS035 as detection-only because
+mdsmith had no equivalent for in-document
+heading TOCs — the common case of those tokens.
+`<?catalog?>` lists *other* files matching a
+glob, so it is the right replacement only on
+index pages, not on the per-document TOCs
+authors usually want.
+
+mdsmith already has the generated-section
+machinery needed here.
+[`<?catalog?>`][catalog] and
+[`<?include?>`][include] each read a directive
+body, compute content, and emit it between
+`<?name ...?>` and `<?/name?>` markers. A rule
+keeps the content current on `mdsmith fix`.
+See the [generated-section archetype][gensection]
+for the shared mechanics.
+
+[catalog]: ../internal/rules/MDS019-catalog/README.md
+[include]: ../internal/rules/MDS021-include/README.md
+[gensection]: ../docs/background/archetypes/generated-section/README.md
+
+### Why both pieces in one plan
+
+The `<?toc?>` directive is only useful to
+MDS035 if MDS035 knows to rewrite its detected
+tokens to it; conversely, MDS035's auto-fix is
+only possible once `<?toc?>` exists. Splitting
+the work across two plans would leave one half
+dead code until the other lands.
+
+## Design
+
+### Directive syntax
+
+```text
+<?toc
+min-level: 2
+max-level: 3
+?>
+- [First heading](#first-heading)
+  - [Subheading](#subheading)
+- [Second heading](#second-heading)
+<?/toc?>
+```
+
+Parameters (all optional):
+
+| Name        | Type | Default | Description                                         |
+|-------------|------|---------|-----------------------------------------------------|
+| `min-level` | int  | `2`     | Lowest heading level to include (1–6)               |
+| `max-level` | int  | `6`     | Highest heading level to include (1–6, ≥ min-level) |
+
+`min-level: 2` excludes the document title by
+default, matching what Python-Markdown's `[TOC]`
+produces with default settings.
+
+### Generated content
+
+A nested unordered list, one item per heading
+in source order. Each item links to a
+GitHub-flavor slug of the heading text:
+
+- Lowercase the heading plain text.
+- Replace spaces with `-`.
+- Strip characters outside `[a-z0-9-_]`.
+- Disambiguate repeats by appending `-1`, `-2`, …
+  in source order (matching goldmark-meta /
+  GitHub's behavior).
+
+Indentation is `<?listindent?>`-aware: use the
+same per-level indent as MDS016 (two spaces by
+default).
+
+List items respect the heading structure, not
+the raw level. Given H2 → H4 → H2, the tree is:
+
+```text
+- [h2a](#h2a)
+  - [h4](#h4)
+- [h2b](#h2b)
+```
+
+…not a flat list keyed on absolute level.
+
+### Rule: MDS036 (toc)
+
+- ID: `MDS036`
+- Name: `toc`
+- Category: `meta`
+- Default: enabled (generated sections are
+  enabled by default for the project; users opt
+  out by removing the directive)
+- Fixable: yes
+
+`Check` diff-compares the body between the
+markers against the regenerated output.
+`Fix` rewrites the body.
+
+Use the shared `internal/archetype/gensection`
+engine that `catalog` already uses. New
+directive logic lives in
+`internal/rules/toc/`.
+
+### MDS035 auto-fix
+
+MDS035 becomes a `FixableRule`. On `Fix`, for
+each matched directive line inside a paragraph:
+
+- `[TOC]` (unresolved), `[[_TOC_]]`, `[[toc]]`,
+  `${toc}` → replace the single line with:
+
+  ```text
+  <?toc
+  ?>
+  <?/toc?>
+  ```
+
+  The empty YAML body means "use defaults". A
+  trailing newline separator is inserted if the
+  surrounding paragraph would otherwise fuse
+  the directive into adjacent text.
+
+- `[TOC]` with a matching link reference
+  definition → leave untouched (already
+  suppressed by Check; must stay out of Fix).
+
+The Fix output is plain `<?toc?>...<?/toc?>`
+with an **empty** body. MDS036 runs in a
+subsequent pass of the same `mdsmith fix`
+invocation (mdsmith already supports
+multi-pass fix, used by MDS019/MDS021) and
+fills in the heading list. If the one-pass
+semantics become brittle, an alternative is for
+MDS035 to call into `gensection` directly and
+emit populated content; start with multi-pass
+and fall back only if needed.
+
+### Interaction with existing rules
+
+- MDS015 (blank line around fenced code /
+  generated sections): the Fix output adds the
+  required blank-line padding around the
+  inserted block.
+- MDS020 (required-structure): no change; TOC
+  blocks are not a required section.
+- MDS019 (catalog), MDS021 (include): orthogonal
+  — different directive names.
+
+### Out of scope
+
+- Custom link anchor format overrides
+  (e.g., non-GitHub slugger). Always GitHub-
+  style for the first release; add a
+  `slugger:` parameter later if needed.
+- Skipping specific headings via frontmatter
+  or attribute syntax. Use `min-level` /
+  `max-level` for now.
+- Rendering `[TOC]` into `<?toc?>` with
+  preserved non-default parameters. The four
+  renderer-specific tokens have no shared
+  parameter surface; always emit the default
+  `<?toc?>`.
+
+## Tasks
+
+1. Create the `<?toc?>` directive in a new
+   `internal/rules/toc/` package using the
+   shared `internal/archetype/gensection.Engine`
+   (same engine as `catalog`). Add a slug
+   helper in `internal/mdtext/` or the toc
+   package. Register as MDS036 in category
+   `meta`, enabled by default, `FixableRule`.
+2. Add MDS036 fixtures under
+   `internal/rules/MDS036-toc/`: `good/` with a
+   correct body, `bad/` with a stale body to
+   verify Check, `fixed/` with the expected
+   output after Fix. Cover default parameters,
+   custom `min-level`/`max-level`, single-level
+   docs, and deeply nested structures.
+3. Upgrade MDS035 to `FixableRule`: replace
+   matched directive lines with
+   `<?toc?>\n<?/toc?>` blocks preserving
+   surrounding blank lines; leave `[TOC]`
+   untouched when a link-ref definition
+   suppresses Check; add `fixed/` fixtures for
+   each of the four variants.
+4. Update MDS035's diagnostic message to point
+   at `<?toc?>` (MDS036) instead of
+   `<?catalog?>` (MDS019); wording:
+   `unsupported TOC directive \`{token}\`; use \`<?toc?>\` (MDS036)`
+5. Update MDS035 README (message + examples),
+   plan 88 status/deviation note, and the
+   renderer-portability section in
+   [docs/background/markdown-linters.md][lnt].
+6. Update the generated-section archetype doc
+   to list `toc` alongside `catalog` and
+   `include`.
+7. Verify multi-pass `mdsmith fix` end-to-end:
+   starting from a document containing `[TOC]`,
+   a single `fix` run must yield a populated
+   `<?toc?>...<?/toc?>` block. Add an
+   integration test asserting this.
+
+[lnt]: ../docs/background/markdown-linters.md
+
+## Acceptance Criteria
+
+- [ ] `<?toc?>...<?/toc?>` in a document
+      regenerates on `mdsmith fix` with a
+      nested list of headings linked to
+      GitHub-style slugs
+- [ ] `min-level` and `max-level` parameters
+      gate which headings appear
+- [ ] `<?toc?>` with a stale body produces
+      an MDS036 diagnostic on `check`
+- [ ] MDS035 `fix` rewrites `[TOC]`,
+      `[[_TOC_]]`, `[[toc]]`, and `${toc}`
+      on their own line to
+      `<?toc?>\n<?/toc?>` blocks
+- [ ] MDS035 `fix` leaves `[TOC]` untouched
+      when a matching link reference
+      definition is present
+- [ ] A single `mdsmith fix` run converts a
+      source containing `[TOC]` into a source
+      containing a populated `<?toc?>` block
+- [ ] Merge driver regenerates `<?toc?>`
+      bodies on conflict, same as `<?catalog?>`
+- [ ] MDS035 diagnostic message names
+      `<?toc?>` (MDS036) as the replacement
+- [ ] All tests pass: `go test ./...`
+- [ ] `go tool golangci-lint run` reports
+      no issues