fix(tui): harden Mermaid rendering limits - #42123

What

Keep terminal Mermaid rendering responsive and readable for oversized or malformed diagrams, and fold wide horizontal Markdown flowcharts before they are silently clipped.

Before / After

Before: a long malformed flowchart statement could spend seconds backtracking in edge parsing; near-million-cell canvases could allocate hundreds of megabytes; malformed chained sequence/state transitions became phantom endpoint names; a wide grapheme could extend beyond the canvas; and the four-stage LR diagram from the OpenCode session rendered wider than the message viewport.

After: pathological statements and oversized canvases fail through existing typed syntax/size errors, ambiguous chains are rejected, clipped wide graphemes are omitted, and Markdown flowcharts wider than 120 columns fold vertically by default.

How

• packages/merman/src/flowchart/parser.ts bounds statement length before edge-operator matching.
• packages/merman/src/core/canvas.ts lowers the eager canvas allocation budget and requires a full grapheme to fit before writing it.
• Sequence and state parsers reject extra arrow operators embedded in endpoints.
• packages/merman/src/markdown.ts passes a configurable layoutMaxWidth into flowchart layout, capped by the renderer width.
• Adds focused regressions for each case, including the exact clipped LR diagram.

Scope

This does not replace the eager canvas with a sparse representation or rewrite flowchart edge parsing as a scanner. The conservative limits protect the TUI without expanding this fix into a storage/layout redesign.

Testing

• cd packages/merman && bun run test (294 passing)
• cd packages/merman && bun run typecheck
• git diff --check