test(graphFormats): cover fromMermaid parser error paths

[mellonis]

Summary

Three new tests in the existing fromMermaid error paths describe block, covering parser-error branches added during the v7 emit overhaul that were never exercised:

• Read label with no bracketed list (graphFormats.ts:379)
• Write/move cell-count mismatch (graphFormats.ts:396)
• Backslash-escape branch in stripBrackets (graphFormats.ts:355-356)

Why

Coveralls reported a coverage drop on PR #167 (v7 → master draft) after v7-alpha.1's emit overhaul landed. The new parser error paths were live but untested — locally clean (above the 97/90/95/97 floors in vitest.config.ts), but enough of a delta vs. master that Coveralls' no-drop check failed.

Coverage delta

Metric	Before	After	Δ
Statements	98.29%	98.72%	+0.43
Branches	95.35%	96.13%	+0.78
Functions	100%	100%	—
graphFormats.ts stmts	96.42%	98.8%	+2.38
graphFormats.ts branches	90.62%	94.79%	+4.17

What's NOT covered

Remaining uncovered branches in v7 code are defensive fallbacks not worth synthetic tests:

• graph.ts:173, 209 — fallback returns in parsePatternString / parseWriteSymbolLabel for malformed inputs
• State.ts:463-464 — catch for unbound Reference.ref throw during toGraph walk
• introspection.ts:121 — if (node) guard in cycle detection

Test plan

• npm test — all tests pass (425/425, +3)
• npm run test:coverage — numbers above
• npm run lint — clean
• npm run typecheck — clean