S3 — The two documents stop being able to disagree about the types they both describe

[The-Running-Dev]

Summary

Implements the mirror check: when 03-game-design.md chooses to describe a closed
declaration from 04-engine-specification.md in prose, a mirror-<QualifiedName>
declared region ties the two together, and Test-SpecSet.ps1 now catches drift
between them instead of relying on someone reading both at once.

• Read-SpecSet.ps1 gains general declared-region extraction (<!-- id:declared:start --> … <!-- id:declared:end -->), reporting MalformedRegion for unclosed,
  mismatched, or nested markers and DuplicateRegionId for a repeated id within one
  document. Regions whose id starts mirror- become MirrorObligation records, each
  carrying the set of member names its body mentions (text after the first colon,
  identifier tokens only).
• Test-SpecSet.ps1 gains Get-MirrorFindings: for each obligation, a missing
  declaration or an open one is a finding; for a closed one, a member the declaration
  has but the body doesn't mention (or vice versa) is a finding. SpecFinding carries
  no field for which side is stale (SS11) — Detail only states disagreement.
• 03-game-design.md §3.1 now carries mirror-NeedState and mirror-AttributeState
  regions around the needs and attributes sentences, held against 04 as it stands.
  §3.5's twelve skills are untouched — skills are an open declaration, so nothing
  obligates them and no finding fires there.
• Write-SpecSetReport states the mirror-obligation count checked and never claims
  the two documents are consistent (SS16).

Decisions made that the contract didn't determine

• S3.1/S3.6's "exits 0" is read at the check level (Get-MirrorFindings returns
  zero findings against the real corpus), not tools/Test-SpecSet.ps1's own exit
  code. The script's exit code is still 2 (NotEvaluated/RegisterAbsent) against the
  real corpus — a pre-existing, unrelated gap (no provisional-register region yet;
  that's S5's scope) already flagged before this slice. Requiring literal exit 0 from
  the CLI would mean this slice silently absorbing S5's work, which Out of scope
  forbids.
• DuplicateRegionId is scoped per-document, matching the contract's literal
  "within one document, an id occurs at most once" — not a global cross-document
  uniqueness rule.
• S3.10 (contract scaffold → pointer) needed no change: design/20-contract.md's
  tools/Test-SpecSet.ps1 section was already a pointer with no scaffold block: no
  commit touches that file.

Acceptance criteria

• S3.1 — mirror-NeedState/mirror-AttributeState regions in §3.1, held against
  04 as it stands, zero mirror findings
• S3.2 — deleting wisdom from §3.1 (04 unchanged) produces exactly one finding
  naming AttributeState.wisdom; restoring returns clean. Both directions tested
  against a copy of the real corpus.
• S3.3 — Detail names the member and both document paths; SpecFinding has no
  stale-side field (reflection-tested)
• S3.4 — an obligation naming an open declaration, or one that doesn't exist,
  raises a finding
• S3.5 — §3.5's twelve skills raise no finding of any kind
• S3.6 — malformed markers → MalformedRegion/exit 2; duplicate id →
  DuplicateRegionId/exit 2; no regions → valid, zero obligations
• S3.7 — AST test asserts no check function calls a file cmdlet or another check
  function
• S3.8 — report states the obligation count checked, never claims consistency
• S3.9 — ./docs.ps1 -BuildOnly succeeds (verified). Could not additionally
  confirm the rendered §3.1 list at runtime — docker run's TTY attach fails in this
  sandbox, unrelated to the change; markers are plain HTML comments carrying nothing
  outside the delimiters.
• S3.10 — already a pointer; no scaffold remained to replace

Verified

Not yet run — this section will be replaced by /pr's gate phase.