fix(foreignregion): resolve MDS073 rule-ID collision with slide-structure

[jeduden]

Summary

A scheduled architecture audit (/solid-architecture audit) swept the ~90 files touched since the last checkpoint (6680ff5..2ab4b29) and found one blocker: the foreign-regions: scanner and the MDS073 Slidev slide-structure rule both landed in the same window and independently claimed rule ID MDS073 — one through the rule.Rule registry, the other as an engine-emitted diagnostic that rides along the fix/check pipeline outside it. That makes .mdsmith.yml's per-rule enabled: false ambiguous for MDS073: it reliably disables the registered rule but has no effect on the foreign-region diagnostic.

• Renumbered the foreign-region scanner's diagnostic from MDS073 to MDS074 (the next free ID) in internal/foreignregion/scan.go, and updated every comment/doc that names it (internal/config/foreignregion.go, internal/config/foreignregion_more_test.go, internal/engine/runner.go, internal/fix/fix_foreign_region_test.go, docs/reference/foreign-regions.md).
• Added internal/integration/rule_id_uniqueness_test.go, a contract test that walks every registered rule.Rule.ID() plus foreignregion.RuleID and fails on any duplicate, so a repeat collision fails CI instead of shipping silently. Verified red (collision) before the rename, green after.
• Ran mdsmith fix . to regenerate the <?catalog?> blocks in CLAUDE.md, AGENTS.md, .github/copilot-instructions.md, and docs/reference/index.md that quote foreign-regions.md's front-matter summary.
• Recorded the audit findings in docs/development/architecture-audit.md (one blocker, fixed directly; one nice-to-have — mergedriver.go/runner.go approaching the 1000-line split threshold, no action needed yet) and advanced audit-from: to the current origin/main SHA.

No other blockers or tax findings surfaced in this sweep (no rule-to-rule imports, no reverse-layer imports, no Liskov breaks, no inverted test pyramid, no missing unit tests on a public surface). The one nice-to-have has no actionable fix yet, so no plan file was filed for it.

Test plan

• go build ./...
• go test ./... (all green)
• go vet ./...
• go tool -modfile=tools/go.mod golangci-lint run on touched packages (0 issues)
• mdsmith check . (0 failures)
• TDD: confirmed TestRuleIDsAreUnique fails against the pre-fix MDS073 collision, then passes after the rename

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Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.71%. Comparing base (2ab4b29) to head (385d8e9).

Additional details and impacted files

Components	Coverage Δ
Go	98.70% <ø> (ø)
TypeScript	99.54% <ø> (ø)

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