plan 51: add MDS036 section-size-limits rule

PLAN.md

@@ -17,7 +17,7 @@ footer: |
 | ID  | Status | Title                                                                                                           |
 |-----|--------|-----------------------------------------------------------------------------------------------------------------|
 | 50  | 🔲     | [Redundancy / Duplication Detection](plan/50_redundancy-duplication-detection.md)                               |
-| 51  | 🔲     | [Section-Level Size Limits](plan/51_section-level-size-limits.md)                                               |
+| 51  | ✅     | [Section-Level Size Limits](plan/51_section-level-size-limits.md)                                               |
 | 52  | 🔲     | [Archetype / Template Library for Agentic Patterns](plan/52_archetype-template-library.md)                      |
 | 53  | ⛔     | [Conciseness Scoring](plan/53_conciseness-scoring.md)                                                           |
 | 54  | ⛔     | [Conciseness Metrics Design and Implementation](plan/54_metrics-guide-tradeoffs.md)                             |

cmd/mdsmith/main.go

@@ -40,6 +40,7 @@ import (
 	_ "github.com/jeduden/mdsmith/internal/rules/linelength"
 	_ "github.com/jeduden/mdsmith/internal/rules/listindent"
 	_ "github.com/jeduden/mdsmith/internal/rules/maxfilelength"
+	_ "github.com/jeduden/mdsmith/internal/rules/maxsectionlength"
 	_ "github.com/jeduden/mdsmith/internal/rules/nobareurls"
 	_ "github.com/jeduden/mdsmith/internal/rules/noduplicateheadings"
 	_ "github.com/jeduden/mdsmith/internal/rules/noemphasisasheading"

internal/config/config_test.go

@@ -28,6 +28,7 @@ import (
 	_ "github.com/jeduden/mdsmith/internal/rules/linelength"
 	_ "github.com/jeduden/mdsmith/internal/rules/listindent"
 	_ "github.com/jeduden/mdsmith/internal/rules/maxfilelength"
+	_ "github.com/jeduden/mdsmith/internal/rules/maxsectionlength"
 	_ "github.com/jeduden/mdsmith/internal/rules/nobareurls"
 	_ "github.com/jeduden/mdsmith/internal/rules/noduplicateheadings"
 	_ "github.com/jeduden/mdsmith/internal/rules/noemphasisasheading"

internal/engine/categories_test.go

@@ -26,6 +26,7 @@ import (
 	_ "github.com/jeduden/mdsmith/internal/rules/headingstyle"
 	_ "github.com/jeduden/mdsmith/internal/rules/linelength"
 	_ "github.com/jeduden/mdsmith/internal/rules/listindent"
+	_ "github.com/jeduden/mdsmith/internal/rules/maxsectionlength"
 	_ "github.com/jeduden/mdsmith/internal/rules/nobareurls"
 	_ "github.com/jeduden/mdsmith/internal/rules/noduplicateheadings"
 	_ "github.com/jeduden/mdsmith/internal/rules/noemphasisasheading"

internal/integration/rules_test.go

@@ -32,6 +32,7 @@ import (
 	_ "github.com/jeduden/mdsmith/internal/rules/linelength"
 	_ "github.com/jeduden/mdsmith/internal/rules/listindent"
 	_ "github.com/jeduden/mdsmith/internal/rules/maxfilelength"
+	_ "github.com/jeduden/mdsmith/internal/rules/maxsectionlength"
 	_ "github.com/jeduden/mdsmith/internal/rules/nobareurls"
 	_ "github.com/jeduden/mdsmith/internal/rules/noduplicateheadings"
 	_ "github.com/jeduden/mdsmith/internal/rules/noemphasisasheading"
@@ -129,6 +130,7 @@ func applySettingsToRule(
 }
 
 func TestRuleFixtures(t *testing.T) {
+	primeDirectoryStructureWarnOnce(t)
 	dirs := discoverFixtureDirs(t)
 
 	for _, dir := range dirs {
@@ -376,6 +378,28 @@ func runFixSingleFile(
 
 // --- shared helpers ---
 
+// primeDirectoryStructureWarnOnce fires MDS033's "no allowed patterns"
+// sync.Once warning up front. Without this, test cleanup in
+// applySettingsToRule leaves MDS033 in a configured-but-empty state,
+// so the warning would fire the first time any post-MDS033 fixture
+// calls checkAllRules, producing a spurious diagnostic.
+func primeDirectoryStructureWarnOnce(t *testing.T) {
+	t.Helper()
+	r := rule.ByID("MDS033")
+	if r == nil {
+		return
+	}
+	cr, ok := r.(rule.Configurable)
+	if !ok {
+		return
+	}
+	require.NoError(t, cr.ApplySettings(map[string]any{"allowed": []any{}}))
+	f, err := lint.NewFile("prime.md", []byte("# x\n"))
+	require.NoError(t, err)
+	_ = r.Check(f)
+	require.NoError(t, cr.ApplySettings(cr.DefaultSettings()))
+}
+
 func discoverFixtureDirs(t *testing.T) []string {
 	t.Helper()
 	dirs, err := filepath.Glob("../../internal/rules/MDS*-*")

internal/rules/MDS036-max-section-length/README.md

@@ -0,0 +1,109 @@
+---
+id: MDS036
+name: max-section-length
+status: ready
+description: Section length must not exceed per-level or per-heading limits.
+---
+# MDS036: max-section-length
+
+Section length must not exceed per-level or per-heading limits.
+
+- **ID**: MDS036
+- **Name**: `max-section-length`
+- **Status**: ready
+- **Default**: disabled
+- **Fixable**: no
+- **Implementation**:
+  [source](./)
+- **Category**: heading
+
+A section spans from its heading line up to (but not including) the next
+heading line of any level, or the end of file. Nested subsections are
+measured separately from their parent, so the limit applies to the
+content written directly under each heading.
+
+## Settings
+
+| Setting       | Type | Default | Description                                        |
+|---------------|------|---------|----------------------------------------------------|
+| `max`         | int  | 0       | Default line limit; zero disables the global rule. |
+| `per-level`   | map  | `{}`    | Map from heading level (1-6) to line limit.        |
+| `per-heading` | list | `[]`    | Regex patterns with per-heading line limits.       |
+
+Lookup order for a heading: `per-heading` (first matching regex wins),
+then `per-level`, then `max`. A resolved limit of zero disables the
+check for that heading.
+
+## Config
+
+Enable with a default limit:
+
+```yaml
+rules:
+  max-section-length:
+    max: 100
+```
+
+Per-level and per-heading overrides:
+
+```yaml
+rules:
+  max-section-length:
+    max: 100
+    per-level:
+      1: 200
+      2: 80
+    per-heading:
+      - pattern: "^Changelog$"
+        max: 500
+```
+
+Disable:
+
+```yaml
+rules:
+  max-section-length: false
+```
+
+## Examples
+
+### Good
+
+<?include
+file: good/default.md
+wrap: markdown
+?>
+
+```markdown
+# Title
+
+Short section within the limit.
+
+## Subsection
+
+Also short. Each section is bounded by the next heading of any level.
+
+## Another
+
+Stays under the limit.
+```
+
+<?/include?>
+
+### Bad
+
+<?include
+file: bad/default.md
+wrap: markdown
+?>
+
+```markdown
+# Too Long
+
+line a
+line b
+line c
+line d
+```
+
+<?/include?>

internal/rules/MDS036-max-section-length/bad/default.md

@@ -0,0 +1,14 @@
+---
+settings:
+  max: 3
+diagnostics:
+  - line: 1
+    column: 1
+    message: 'section "# Too Long" too long (6 > 3)'
+---
+# Too Long
+
+line a
+line b
+line c
+line d

internal/rules/MDS036-max-section-length/bad/per-level.md

@@ -0,0 +1,17 @@
+---
+settings:
+  max: 0
+  per-level:
+    2: 2
+diagnostics:
+  - line: 3
+    column: 1
+    message: 'section "## Overflowing" too long (5 > 2)'
+---
+# Overview
+
+## Overflowing
+
+a
+b
+c

internal/rules/MDS036-max-section-length/good/default.md

@@ -0,0 +1,15 @@
+---
+settings:
+  max: 10
+---
+# Title
+
+Short section within the limit.
+
+## Subsection
+
+Also short. Each section is bounded by the next heading of any level.
+
+## Another
+
+Stays under the limit.

internal/rules/MDS036-max-section-length/good/per-level.md

@@ -0,0 +1,18 @@
+---
+settings:
+  max: 0
+  per-level:
+    2: 4
+---
+# Overview
+
+Top-level has no limit because `max` is zero and level 1 is not listed.
+Only level-2 sections must stay within four lines.
+
+## Short
+
+Two lines of text.
+
+## Also Short
+
+Two more lines.

internal/rules/index.md

@@ -52,4 +52,5 @@ row: "| [{id}]({filename}) | `{name}` | {status} | {description} |"
 | [MDS031](MDS031-unclosed-code-block/README.md)                | `unclosed-code-block`                | ready     | Fenced code blocks must have a closing fence delimiter.                                 |
 | [MDS032](MDS032-no-empty-alt-text/README.md)                  | `no-empty-alt-text`                  | ready     | Images must have non-empty alt text for accessibility.                                  |
 | [MDS033](MDS033-directory-structure/README.md)                | `directory-structure`                | ready     | Markdown files must exist only in explicitly allowed directories.                       |
+| [MDS036](MDS036-max-section-length/README.md)                 | `max-section-length`                 | ready     | Section length must not exceed per-level or per-heading limits.                         |
 <?/catalog?>