feat(explorer): v0.8.5 Explorer relationship navigation — traverse, impact, lineage

[tcballard]

Summary

Implements rac/roadmaps/v0.8.x-explorer/v0.8.5-explorer-relationship-navigation.md.

Adds:

• A relationship view (g from a context view, or /relationships <ref>): an artifact's outgoing relationships, its impact ("what depends on this?"), and its lineage (Supersedes / Superseded By)
• Graph traversal — connected artifacts are selectable, opening their context view, from which g reopens relationships; Esc unwinds
• All rendered from the loaded repository model; Explorer infers nothing (ADR-016)

Roadmap / ADR Trace

Roadmap: rac/roadmaps/v0.8.x-explorer/v0.8.5-explorer-relationship-navigation.md (contract pinned in the first commit)

Relevant ADRs and designs:

• ADR-015 — relationship intelligence stays in Core
• ADR-016 — relationships are authored in artifacts; Explorer reads, never edits or infers
• ADR-020 — the requirement → implements/constrains/expresses traceability graph
• DESIGN-knowledge-graph

Scope

Included

• rac.explorer.state.RelationshipLink / RelationshipsView; ExplorerAdapter.relationships_view(path) built from the model's Relationship objects
• RelationshipScreen — Relationships / Impact / Lineage sections plus a selectable list of connected (resolved) artifacts; g binding on the context view; /relationships <ref> routing (resolves the ref, then opens the view)

Excluded (deliberately)

• Manual relationship editing or inference (ADR-016, ADR-015)
• Created/Updated lineage timestamps — not modelled by Core; lineage covers Supersedes / Superseded By only
• Any graphical canvas or web-style graph editor (Initiative 5) — terminal-readable text only
• No Core/service changes; no JSON contract movement

Product / Architecture Decisions

• The view is derived entirely from repository.relationships: outgoing edges are relationships whose source is this artifact (resolved to a title, or marked ✗ unresolved); impact is relationships whose resolved target is this artifact (the dependents); lineage filters supersedes in both directions. A test asserts the impact set equals the model's incoming-resolved references.
• Traceability (ADR-020) needs no separate engine — it is the same outgoing/incoming traversal followed from a requirement.
• The connected-artifact list is keyed by option index (a path can appear via several edges; OptionList ids must be unique), reusing the pattern from health/recommendations.
• g (context) and /relationships <ref> both open the same screen; the command resolves the ref with rac resolve semantics first, surfacing not-found / duplicate like /open.

User-Facing Contract

CLI / Keys

rac explorer
# context view:  g → relationships;  Enter on a connected artifact → its context → g → …
# /relationships adr-015

Human Output (interactive)

ADR-015: Consumers before Interfaces

Relationships
  none declared

Impact (what depends on this)
  ← Explorer Command Surface (Related Decisions)
  ← Explorer Health (Related Decisions)

Lineage
  no recorded supersession

JSON Output

No changes; no new contracts.

Exit Codes

Unchanged: 0 session quit · 2 not a directory or missing explorer extra.

Verification

Ran

python -m pytest                      # 758 passed
python -m ruff check src/ tests/
python -m ruff format --check src/ tests/
python -m mypy src/
rac validate rac/                     # exit 0
rac relationships rac/ --validate     # exit 0
rac review rac/                       # no priority 1–2 findings

Covered

• Adapter: outgoing resolves to the target artifact path; impact is the set of dependents (asserted equal to the model's incoming-resolved references for every artifact); unresolved targets are non-navigable and marked ✗; None before a load
• Screens: g opens the relationship view (Relationships/Impact/Lineage sections), selecting a connected artifact opens its context view and Esc returns — traversal both ways; /relationships <ref> opens it for a resolved ref
• Registry: relationships discoverable; /help lists all 10 entries

Review Path

1. rac/roadmaps/v0.8.x-explorer/v0.8.5-explorer-relationship-navigation.md — the pinned contract
2. src/rac/explorer/adapter.py — relationships_view (outgoing / impact / lineage derivation)
3. src/rac/explorer/screens/relationships.py — the screen and traversal
4. src/rac/explorer/screens/context.py, commands.py, screens/command.py — g binding and /relationships
5. tests/ — adapter, app, commands
6. docs/cli.md, CHANGELOG.md

Notes For Reviewer

• Stacked on v0.8.2–v0.8.4 (PRs feat(explorer): v0.8.2 Explorer health — health view, four areas, navigable attention #53–feat(explorer): v0.8.4 Explorer action workflows — editor launch, guided import, export #55); until those merge this PR's diff also shows their commits. Merging in order narrows each.
• Closes the v0.8.1/v0.8.4 deferral of Open Related Artifact / Show Relationship Impact.

Implementation Process

Implemented with AI assistance under the roadmap contract.

Final scope, review, and acceptance decisions were made by the maintainer.