refactor(#1771): ADR-1769 Phase 1 — STATE.md Transition Module substrate + beginPhase (#1775)

* refactor(#1771): ADR-1769 Phase 1 — STATE.md Transition Module substrate + beginPhase

Lands the Phase 1 substrate per ADR-1769:

- src/state-transition.cts (new Module):
  - Field-classification table (FieldClass enum + FIELD_CLASSIFICATION rows)
  - STATE_MD_SECTIONS constants block
  - Pure transitionCore(content, intent, deps) dispatch
  - beginPhase intent implementation (first-time + #3127 resume paths)

- src/state.cts:cmdStateBeginPhase — collapses ~190 lines to a thin
  dispatch onto transitionCore via readModifyWriteStateMd. The lock,
  no-op write guard, and #1230 post-sync delta heuristic stay in the
  RMW seam; the body-mutation policy moves to transitionCore.

- tests/state-transition.test.cjs (24 tests):
  - Substrate invariants (table enum, section constants)
  - Characterization: 6 first-time body field updates
  - Characterization: 5 #3127 idempotency-guard resume behaviors
  - Characterization: 3 Current Position section mutations
  - Characterization: Current focus body text line (#1104)
  - Property (RULESET.TESTS.property-based-testing): beginPhase status
    propagation + FIELD_CLASSIFICATION own-property contract
  - Resume Current Position mutation (preserves Plan/Phase/Status)

No external behavior change. Full state.test.cjs regression (177 tests)
plus bug-3127/#3242/#905/#948 pass. Property tests surfaced two
pre-existing quirks (state-document.cjs greedy \s* on whitespace-only
field values; Object.prototype method leakage on FIELD_CLASSIFICATION
lookups for strings like 'toString') — documented in test comments;
fix-out-of-scope for Phase 1.

Closes #1771

* refactor(#1771): ADR-1769 Phase 1 codex review corrections

Addresses 3 blocking findings from codex gpt-5.5/high review:

1. FIELD_CLASSIFICATION shape (state-transition.cts):
   - Was flat FieldClass enum (collapsed source + preservation)
   - Now two-column {source, preservation} rows per ADR-1769 §4
   - Added missing fields verified via Memtrace against
     buildStateFrontmatter (state.cts:1633-1653): gsd_state_version,
     last_updated, last_activity_desc, progress.{total_phases,
     completed_phases, total_plans, completed_plans, percent}
   - Field 2-7 preservation dispatch can now consult the table

2. Prototype-pollution hardening (state-transition.cts):
   - Table is now Object.freeze(Object.assign(Object.create(null), {...}))
   - getFieldClassification() helper uses Object.hasOwn; returns null
     for inherited prototype methods (toString/valueOf/__proto__)
   - Old code: FIELD_CLASSIFICATION['toString'] returned the function

3. STATE_MD_SECTIONS aligned to canonical template:
   - Verified against gsd-core/templates/state.md via Memtrace
   - Was: 8 entries including non-template sections (## Session,
     ## Decisions, ## Operator Next Steps, ## Session Log,
     ## Roadmap Evolution)
   - Now: 6 canonical top-level sections (## Project Reference,
     ## Current Position, ## Performance Metrics, ## Accumulated
     Context, ## Deferred Items, ## Session Continuity)

Also: beginPhase now consults getFieldClassification() per touched
field (codex finding: 'table not consulted by transitionCore').
Unknown fields raise immediately — adding a field without a table
row is caught at runtime.

Repo-hygiene catches from gsd-test (not node --test, which missed
these):
- gsd-core/bin/lib/state-transition.cjs added to eslint.config.mjs
  ignore list (ADR-457 tsc-generated)
- docs/INVENTORY-MANIFEST.json regenerated via
  node scripts/gen-inventory-manifest.cjs --write

Property test for Object.prototype leakage tightened to verify
getFieldClassification() returns null for toString/valueOf/__proto__.

Ref #1771

* fix(#1771): cast Object.create(null) to satisfy @typescript-eslint/no-unsafe-assignment

ESLint CI failed on src/state-transition.cts:72:14 — Object.create(null)
returns `any`, which leaked through Object.assign to the typed
`FIELD_CLASSIFICATION` declaration. Adding an explicit cast to
`Record<string, FieldClassification>` eliminates the unsafe-assignment
while preserving the null-prototype protection codex review recommended.

gsd-test: 21888/21888 PASS.

* fix(#1771): add 'see #1771' to allow-test-rule exemption per ADR-456

CI lint-allow-test-rule-refs failed: 'New allow-test-rule exemption
without an issue ref — add `see #NNN` per ADR-456'. Updated comment
on tests/state-transition.test.cjs to reference the Phase 1 issue.

* fix(#1771): remove unnecessary allow-test-rule exemption

The exemption was added speculatively. The test file does not use
readFileSync + .includes()/.match()/.startsWith() on source content —
it calls transitionCore() with string literals and verifies results
via stateExtractField() and array .includes() on the updated[] array.
No exemption needed.

eslint.config.mjs

@@ -76,6 +76,7 @@ export default tseslint.config(
       'gsd-core/bin/lib/context-utilization.cjs',
       'gsd-core/bin/lib/artifacts.cjs',
       'gsd-core/bin/lib/assumption-delta.cjs',
+      'gsd-core/bin/lib/state-transition.cjs',
       'gsd-core/bin/lib/command-arg-projection.cjs',
       'gsd-core/bin/lib/clock.cjs',
       'gsd-core/bin/lib/ui-safety-gate.cjs',