M9-03: Add Revit-openability validation path

[DrunkOnJava]

Created from the roadmap audit in TODO.md.

Source: https://github.com/DrunkOnJava/rvt-rs/blob/main/TODO.md#L1055
Tracking commit: 1ec8105

Scope

Labels: priority:P3, type:research, area:writer

• Define how to validate modified files without requiring every
  contributor to have Revit.
• Explore:
  • community validation volunteers
  • self-hosted Windows runner with licensed Revit, if legally allowed
  • checksum/structural invariants as partial substitute

Acceptance criteria:

• Documented decision on whether Revit-in-the-loop validation is in
  scope.

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• The TODO acceptance criteria for this section are implemented or explicitly superseded by a linked decision.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 00e1efc.

Added docs/decisions/ADR-003-revit-openability-validation.md, which defines Revit-openability as an optional high-confidence validation tier rather than a baseline public-CI requirement. The ADR documents the community validation checklist, conditions for any self-hosted Revit runner, structural substitutes when Revit is unavailable, and the distinction between structural, semantic, and Revit-openability claims. ADR-002 now references ADR-003 for semantic-write validation evidence.

Verification:

• Local: git diff --check and stub-pattern scan passed.
• GitHub: docs-only changelog check for 00e1efc is green; main CI is path-ignored for docs-only changes.