M9-02: Design semantic write API, do not ship early

[DrunkOnJava]

Created from the roadmap audit in TODO.md.

Source: https://github.com/DrunkOnJava/rvt-rs/blob/main/TODO.md#L1037
Tracking commit: 1ec8105

Scope

Labels: priority:P2, type:research, area:writer

• Draft design for field-level edits:
  • supported field types
  • transaction model
  • validation
  • backup/atomic write
  • Revit-openability checks
• Do not implement user-facing semantic writes until reader coverage is
  strong enough to avoid corrupting files.

Acceptance criteria:

• ADR documents risks and proposed API.
• Any prototype is behind an explicit experimental feature.

Definition of done

• The TODO acceptance criteria for this section are implemented or explicitly superseded by a linked decision.
• Tests, fixtures, or documentation are added at the level appropriate to the change risk.
• User-facing behavior and limitations remain honest in README/docs/viewer messaging.
• Relevant CI checks pass before the issue is closed.

[DrunkOnJava]

Completed in cb7b4fd.

Added docs/decisions/ADR-002-semantic-write-api-gate.md, which documents the semantic write API design and explicitly gates any future prototype behind experimental-semantic-write plus an opt-in CLI/build surface. The ADR covers supported field types, transaction model, validation tiers, backup/atomic write behavior, Revit-openability strategy, risks, and rejected alternatives. README and the user guide now link to the ADR and continue to state that stable semantic editing is not shipped.

Verification:

• Local: git diff --check and stub-pattern scan passed.
• GitHub: docs-only changelog check for cb7b4fd is green; the main CI workflow is path-ignored for docs-only changes.