test(coverage): cover remaining reachable v7 branches

[mellonis]

Summary

Follow-up to #171 — three more tests for v7 lines that #171's parser-error tests didn't reach:

1. State.toGraph unbound-Reference catch in WRAPPER context (State.ts:463-464). There are two try/catch blocks for unbound Reference.ref in toGraph — one in the non-wrapper branch (already tested) and one in the wrapper-context branch (lines 463-464). The new test wraps the unbound-Ref bare via withOverriddenHaltState(...) to exercise the second branch.
2. parseWriteSymbolLabel fallback return (graph.ts:209) — defensive return-as-is for unrecognized labels. Symmetric with the parsePatternString fallback that test(graphFormats): cover fromMermaid parser error paths #171 added.
3. parsePatternString blank-marker ?? cell fallback (graph.ts:164) — defensive return when alphabets[tapeIx] is missing.

Why

#171 narrowed the Coveralls drop on PR #167 (v7 → master) from -0.5% to less, but didn't fully clear it (Coveralls reported Coverage decreased (-0.5%) to 97.805%). This PR adds the remaining reasonably-reachable branches.

Coverage delta

Metric	Pre-#171	After #171	After this PR	Total Δ
Statements	98.29%	98.72%	99.14%	+0.85
Branches	95.35%	96.13%	96.71%	+1.36
Lines	98.32%	—	99.21%	+0.89
State.ts stmts	99.11%	99.11%	100%	+0.89
graph.ts stmts	96.96%	100%	100%	+3.04

Master's Coveralls baseline was ~98.305% lines; v7 is now at 99.21% — should land above master and clear the red.

What's NOT covered (intentional)

Two genuinely defensive branches not worth synthetic tests:

• graphFormats.ts:344 — throw "malformed bracketed list" inside a stripBrackets closure that's only called with slices the parser itself constructs from regex-matched […] blocks. Not reachable via the public fromMermaid API.
• introspection.ts:121 — if (node) guard in cycle detection for a node that's in the color map but not in graph.nodes. Can't happen with toGraph-produced graphs.

Test plan

• npm test — 431/431 pass (+3 from test(graphFormats): cover fromMermaid parser error paths #171's 428)
• npm run test:coverage — numbers above
• npm run lint — clean
• npm run typecheck — clean