Render Mermaid diagrams in the TUI

[acking-you]

Closes #621

Summary

• render completed mermaid code fences as bounded Unicode diagrams in the TUI transcript
• support common flowchart, state, class, entity-relationship, and sequence diagram syntax
• split parsing, owned models, label normalization, layout, and canvas painting into focused modules
• preserve the ordinary source code block for unsupported, over-budget, or too-wide diagrams
• recompute presentation from semantic transcript content on resize, making diagram to source fallback and restoration reversible
• sanitize terminal control characters and bound parser, graph, sequence, label, and canvas allocations

The width-aware fallback follows the UX established by Grok Build's terminal Mermaid renderer: render the diagram when the TUI can carry it, otherwise keep the original source readable.

Example

```mermaid
flowchart LR
  A[Request] -->|validate| B[Response]
```

At a wide transcript width this renders as terminal-native diagram art. Shrinking below the diagram's required width restores the fenced Mermaid source; widening the terminal renders the diagram again.

Verification

• zig fmt --check src/
• zig build -Doptimize=ReleaseSafe
• focused ReleaseSafe Mermaid suite: 19/19 tests passed
• ./scripts/check-public-surface.sh
• deterministic tmux TUI E2E: 1/1 passed with 7 assertions

The E2E covers the complete requested lifecycle in one real model turn: an open fence stays source, the closed fence renders at 90 columns, a resize to 20 columns falls back to source, and a resize back to 90 columns restores the diagram. It also verifies one model request, a live pane, clean stderr, and a usable composer after completion.

Classification

type: feature