One parse / one write path + write-path consistency check

[Firehed]

Slice

• Slice: S3.4 — One parse / one write path + consistency check
• Plan step: Plan 0002 Step 3a(iv) (docs/architecture/0002-execution-plan.md)
• RFC: 0001-foundational-architecture.md §4.3 (Read/Write Segregation — exactly one write path), §5.2 (SymbolSink write contract)

What this does

Collapses the sink's double write into one parse feeding both stores, and adds a
consistency guard so the two structures cannot silently diverge.

Before, DocumentSymbolSink::write() registered class metadata and indexed symbols
via two independent code paths, each of which parsed the document (the content-keyed
parse memo made this one actual parse, but two parse calls / two extraction
entries). Now the sink parses once and feeds that single AST to both the class-lookup
registration and the symbol index:

• DocumentIndexer::indexParsed(TextDocument, array $ast) indexes a supplied AST
  instead of reparsing; index() is now parse() → indexParsed(). The sink drives
  the index through indexParsed, so the indexer no longer reparses.
• The two stores stay distinct structures (Plan 0002 §5.5 permits this — ClassInfo
  for lookup, lightweight Symbols for enumeration/search), but are written
  transactionally from the one AST.

Because the Step P parity harness compares only observable outputs, an internal
divergence between the two structures could pass parity. assertStoresAgree() guards
the invariant directly: every class-like registered for lookup must also be present in
the symbol index, so a name is never resolvable through one surface yet invisible to
the other. The check is one-directional — the index is a legitimate superset (it also
records class-likes declared inside conditional/nested statements, which the top-level
lookup registration does not), so the reverse would false-positive on valid PHP. The
divergence branch is an unreachable LogicException.

This discharges the Teardown-ledger row "Step 2 facade hides today's double write
(§5.5) → Step 3a(iv)": the scaffolding was the double write inside the sink (S3.3
already replaced the illustrative DelegatingSymbolSource with DocumentSymbolSink),
now collapsed.

Behavior-preserving (Step 3a): the Step P parity goldens (class-like lookup,
childrenOf, prefix-search, write-path symbol state) are unchanged and the full suite
is green. The consistency guard is a new internal invariant, so it carries its own
unit coverage rather than a parity golden.

Acceptance criteria

• One parse per didOpen/didChange write, feeding both stores (the sink parses; the indexer no longer reparses — indexParsed)
• The two structures may stay distinct, written transactionally from the same parse (§4.3, Plan 0002 §5.5)
• Consistency check that both stores are written from the same parse and agree (assertStoresAgree)
• Teardown-ledger row (Step 2 double write, §5.5) discharged
• Behavior-preserving: Step P parity goldens unchanged; full suite green

Candidate closes

None — the manifest lists no Closes for S3.4.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.25%. Comparing base (98a7cea) to head (ae52373).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #385      +/-   ##
============================================
+ Coverage     98.23%   98.25%   +0.02%     
- Complexity     1725     1728       +3     
============================================
  Files           111      111              
  Lines          4356     4360       +4     
============================================
+ Hits           4279     4284       +5     
+ Misses           77       76       -1     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.