feat(diagnostics): add strict writing diagnostic contract

[seonghobae]

Scope

Stacked implementation lane for Task 1 of PR #248's committed writing-diagnostics plan. The PR remains Draft and targets design/llm-writing-diagnostics so the governing ADR/design is an explicit parent dependency.

Test-first lineage

The implementation followed an observable RED -> GREEN sequence:

1. test-only head 9f3c6dc219048363ad1699b693f415d32495ffe4 defined the strict contract before production code;
2. workflow run 31553859019 failed because the production module did not exist;
3. the validator implementation made the contract suite green;
4. writingDiagnosticsExports.test.ts then failed because root and framework-independent exports were absent in run 31554191623;
5. root and src/writing-diagnostics exports were added and passed;
6. exact focused coverage exposed hostile-input branches; boundary tests and removal of logically unreachable branches produced 465/465 statements, 21/21 functions, and 114/114 branches;
7. the whole-repository coverage run exposed a pre-existing ADR-index quality-contract regression, which was repaired without weakening the gate;
8. PR docs(adr): define revision-bound host-owned writing diagnostics #248 later added ADR 0028 and the v1 contract errata to resolve strict invalidation and semantic-neutral accessibility; this branch was non-destructively reconciled with exact parent e580aec1961673b49f98e2a62debfb6ce4e48df6 through merge commit 322c838d23de9ba574bcc549ad1e72cd2dafe2fe.

Included behavior

• provider-neutral CwlWritingDiagnostic and provenance contracts;
• finite advisory | important | critical presentation priority;
• frozen default resource ceilings and optional stricter caller ceilings;
• stable redacted error classes for contract, limit, revision, projection, selector, conflict, and lifecycle failures;
• exact own enumerable data-property validation;
• rejection of accessors, inherited fields, symbol fields, extra fields, sparse or structurally hostile arrays, proxies, and duplicate diagnostic IDs;
• exact lowercase SHA-256 revision and strong ETag validation;
• exact inkspan-prosemirror-text version 1 projection validation;
• finite, non-negative, safe-integral W3C text-position selectors;
• bounded host strings, replacement text, provenance, and confidence;
• deep detachment and freezing of every accepted object;
• identical root and React-free source-level exports;
• inherited ADR 0028/errata authority prohibiting category-derived semantic ARIA and requiring full invalidation after every document change.

Authority boundary

This lane performs no semantic language judgment. It does not infer spelling, grammar, tone, clarity, pragmatics, technical quality, or actionability. It contains no keyword/regex/phrase-list fallback, no model/provider/network/database/credential access, no stale-text repair, no editor decoration, no document mutation, and no send gate.

Exact current-head evidence

• exact base: design/llm-writing-diagnostics@e580aec1961673b49f98e2a62debfb6ce4e48df6;
• exact head: 322c838d23de9ba574bcc549ad1e72cd2dafe2fe;
• ancestry compare: ahead 20, behind 0, merge base exactly the current design head;
• focused Writing Diagnostics Contract TDD run 31589881613: success on this exact head;
• no canonical repository CI/Security/Semgrep run is claimed for this merge head yet;
• predecessor-head checks, status and reviews do not transfer.

Remaining gates

• the stacked design PR docs(adr): define revision-bound host-owned writing diagnostics #248 remains Draft and must be accepted first;
• the temporary branch-specific TDD workflow must be removed before integration and the resulting exact head revalidated through canonical repository checks;
• PR docs(adr): define revision-bound host-owned writing diagnostics #248's current exact Security Scan and Semgrep are still pending and are not inherited by this branch;
• fresh current-head automated/human review, zero valid unresolved findings, independent approval and all protected checks remain mandatory;
• release publication remains a separate release-only PR after issue release: reconcile Office and npm versions before next stable registry publication #118 closes.

The PR therefore remains Draft despite the focused current-head run succeeding.

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