fix(renderer): animate mermaid block height when the diagram renders

[roxi3906]

Problem

Streamdown reserves a fixed ~200px placeholder for each mermaid fence and only renders the diagram once the block scrolls near the viewport (IntersectionObserver + idle callback). The rendered SVG's height almost never matches the placeholder, so the moment the diagram appears, the surrounding transcript jumps abruptly.

Change

Adds an install-time DOM enhancement (installMermaidHeightAnimation, wired into installStreamdown) that smooths the swap:

• A MutationObserver discovers [data-streamdown="mermaid-block"] elements inside .agent-markdown-root (covers both static and streaming render modes, since both go through the same DOM).
• A ResizeObserver tracks each block's height. When it changes — placeholder → spinner → rendered SVG, streaming re-renders, window resize — the block's height is pinned to the previous value and transitioned to the new height (180ms, ease) instead of snapping. Inline styles are cleaned up when the transition ends so the block returns to height: auto.
• The first measurement after observe never animates (no animation on initial mount).
• Sub-2px deltas are ignored to avoid animating rounding noise.
• prefers-reduced-motion: reduce skips the animation entirely.
• A height change that lands mid-transition is re-measured when the transition finishes and chains into a follow-up animation instead of snapping.
• Removed blocks are unobserved and any in-flight inline styles are restored; uninstalling mid-animation also restores auto height.

No upstream patch and no changes to streamdown's rendering or to the mermaid block chrome (actions/fullscreen/download all untouched).

Testing

• New unit suite mermaid-height-animation.test.ts (9 tests): placeholder→render animation, initial-measurement skip, delta threshold, mid-transition chaining, observer frame filtering, reduced motion, blocks outside the markdown root, add/remove tracking, uninstall cleanup.
• Full vitest run: 23,695 passed; the only 4 failures (network-enforcement.integration, literature-review kernel, kernel-executor Windows cancellation) reproduce identically on a clean main checkout — pre-existing environment issues (Python version, sandbox network policy), unrelated to this change.
• npm run typecheck and ESLint clean.