fix(analysis): require a sealed snapshot for Architecture, remove fabricated metrics

[parthrohit22]

Summary

Architecture was the one flagship product surface still bypassing the sealed
ri.v1 snapshot contract: it fell back to record.file_tree (unsealed
repository metadata) when no snapshot existed, and it invented
estimated_lines/estimated_complexity from file counts
(max(len(module.files) * 80, 20), a file-count-derived guess) rather than a
real measurement. This PR removes both violations so Architecture behaves
like every other consumer (Dependencies, Review, Insights): a sealed snapshot
or an explicit missing_snapshot/not_computed state, never a fabricated
number.

Linked issue

Closes #217

Roadmap alignment

• Roadmap §23 workstream this advances: W0 / Truth contract — "Every
  shipped claim maps to tested fact/evidence contracts."
• §28 market-fit criterion this moves toward: Trusted output — "users
  verify and cite PARTHA evidence; reported false certainty is rare, measured,
  and corrected."
• Accepted evidence it is real: the 2026-07-27 audit (§7, CQ-H1) identified
  the exact violating lines (architecture.py:133,185, current dev); this
  PR's regression tests (test_architecture_relationships.py,
  test_legacy_read_model_guard.py) fail on the old behaviour and pass on the
  new one, verified locally (see Testing performed).

What changed

Backend

• app/analysis/architecture.py: build_architecture now requires a sealed
  snapshot via SnapshotQueryService.require_sealed_snapshot_for_current_revision
  before building anything (raises NotFoundError → 404, same contract as
  Dependencies/Review/Insights). Removed the record.file_tree fallback and
  the now-dead _persisted_file_paths helper. estimated_complexity/
  estimated_lines are now always "not_computed" for every node
  (module and dependency) instead of a file-count-derived guess.
• app/schemas/architecture.py: ArchNode.estimated_complexity/
  estimated_lines widened to accept an explicit "not_computed" state
  alongside real values, so a future real heuristic can populate them without
  another contract break.
• app/reports/builders.py: dropped the "Size Class" export row that only
  ever printed the fabricated complexity value.
• app/api/routes/analysis.py: updated the /architecture OpenAPI 200
  example to reflect that 200 now always means a sealed snapshot backs the
  response (no more "no sealed snapshot" diagnostic in a 200 example).
• tests/test_legacy_read_model_guard.py: extended with a new AST guard,
  test_analytical_consumers_never_read_repository_file_tree, that fails if
  any file under app/{analysis,graph,review,insights,ai,reports} reads
  .file_tree — the contract test the issue's acceptance criteria asked for.

Frontend

• useArchitecture.ts / ArchitecturePage.tsx: added the same noSnapshot
  empty state Dependencies already has ("No sealed snapshot yet — Run
  analysis") instead of the page silently reaching !architecture.model with
  no useful next action.
• ArchitectureNode.tsx, NodeInspector.tsx: removed the complexity badge
  and the Complexity/Est. Lines inspector stats — they only ever displayed the
  fabricated value. The accessible name no longer claims a complexity that
  wasn't measured.
• layout.ts, HeatmapControls.tsx, shared/types/architecture.ts: removed
  the complexity and size heatmap modes (zero real signal existed for
  either); critical is recomputed from dependents-count and file-count only,
  both real graph facts.

Acceptance criteria completed

• Architecture returns an explicit missing-snapshot state (404, matching
  Dependencies/Review/Insights) when no sealed ri.v1 snapshot exists — no
  200 fallback graph.
• No Architecture payload can contain a numerical estimated_lines/
  estimated_complexity value not backed by a snapshot fact — every node now
  reports "not_computed".
• A new contract test fails if any analytical consumer reads
  RepositoryRecord.file_tree or repo_metadata for analytical output.
• productSurfaces.test.tsx/accessibility.test.tsx still pass (verified
  — neither asserts on Architecture-specific fields, unaffected).
• npm run build:frontend and backend pytest pass; test_openapi_contract.py
  passes unchanged (no field was removed, only widened).

Testing performed

cd apps/backend && PYTHONPATH=<worktree>/apps/backend .venv/bin/python -m pytest
  → 731 passed, 3 skipped (PostgreSQL/Redis-gated, expected locally), exit 0

npm --prefix apps/frontend run test -- --run
  → Test Files  33 passed (33) / Tests  177 passed (177)

npm run lint:frontend
  → clean, no errors

npm run build:frontend
  → tsc -b && vite build succeeded, no type errors

git diff --check
  → clean, no whitespace errors

Screenshots

Not applicable (empty-state text/badge changes only; no new visual surface —
the "No sealed snapshot yet" state reuses the existing EmptyState component
already visible on the Dependencies page).

Security and data considerations

None. No auth/owner-scoping change (the 404 path reuses the same owner-scoped
require_sealed_snapshot_for_current_revision every other consumer already
uses). No new data is stored or logged. No migration.

Dependencies and blocked work

None.

Scope changes or remaining work

None — all acceptance criteria from #217 are complete. Follow-up note: a real
size/complexity heuristic (with a named formula and truth class) could
replace not_computed in a future issue; this PR deliberately does not add
one (explicit non-goal in the issue).

Contributor checklist

• This PR targets dev
• I claimed the issue and had it assigned or acknowledged before starting substantial work
• The branch was created from an up-to-date upstream/dev
• The branch is rebased on the latest upstream/dev
• This PR addresses one clearly scoped issue
• This PR advances a §23 workstream toward a §28 market-fit criterion (Roadmap alignment filled)
• Every acceptance criterion I claim as complete is actually complete
• Relevant tests pass
• Documentation is updated for any user-visible change (none required — no public doc described the old fallback behaviour as current)
• No secrets, credentials, local env files, or generated artifacts are included
• No unrelated files were changed
• Closing syntax (Closes) is used only because the issue is fully resolved
• Dependencies and follow-up work are linked