chore(#455,#498): enforce src/core boundaries and catch openMenu extension classes

[BorisTyshkevich]

What & why

Three low-priority gate/test-infra fixes with no user-facing behaviour change.

Closes #455 — build/check-boundaries.mjs had no src/core rule, so the invariant src/application/main-surface.ts documents ("src/core/ must never import src/workspace/") was not mechanically enforced. Adds the RULES entry, deliberately narrower than "no imports from any other layer" so the documented legacy type-only core/saved-io.ts → editor/spec-editor.types.js dependency keeps passing. tests/unit/dashboard-boundaries.test.js gains a real-tree check plus a sabotage probe that plants a throwaway forbidden import inside src/core, asserts the detector flags it, and removes it in a finally — so the clean-tree assertion cannot go vacuous.

Closes #498 — the typography contract only scanned for class: '…' hyperscript literals, so every class supplied through openMenu/openConfirmMenu options (menuClass, extraClass, goClass, cancelClass) was invisible to it. Adds a separate openMenu extension classes have CSS rules test that walks all of src/ui/**/*.ts recursively (not a curated list, so new menu consumers are covered automatically), splits whitespace-separated class groups, and asserts each has a rule in src/styles.css. The existing generic scanner is untouched, so .file-menu/.fm-item/.fm-section stay covered. A sabotage test proves removing one such rule fails the contract.

The new scanner immediately surfaced one real gap: .dash-style-item (on every layout-picker row via buildLayoutMenu) had no rule at all. Added one — see the note below.

#435 needed no code change and is closed separately with the evidence. tests/unit/filter-bar.test.ts no longer exists: #459 renamed it to tests/unit/variable-bar.test.ts and replaced the four raw 0x00 bytes with a single escaped GROUP_KEY constant matching production. Verified independently — no file under src/ or tests/ on main contains a NUL byte (only PNGs do, as expected).

Reviewer note on the one CSS line

.dash-style-item { justify-content: space-between; } is a functional declaration added to satisfy a test gate, which deserves scrutiny. It is verifiably inert: every dash-style-item row sets trailing unconditionally (dashboardStyleKeyCaps(shortcut)), and .fm-trailing's margin-left: auto already absorbs all free space, so space-between has nothing left to distribute. It documents the row's existing intent rather than changing layout. Flagged deliberately rather than buried.

Checklist

• npm test passes (the per-file coverage gate is non-negotiable)
• Tests added/updated in the same change as the code
• npm run build succeeds (single-file dist/sql.html)
• Layers kept honest: pure logic in src/core/, network in src/net/ (injected fetch), DOM in src/ui/
• No new runtime dependency
• README / CHANGELOG.md ([Unreleased]) updated if behavior or the deployed surface changed
• Reconciled affected tracked work (roadmap Roadmap to 1.0.0 #68, the issue body, ADR/CHANGELOG) if this change reshaped it

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