perf(tui): skip prefix rebuild on steady-state refresh and cache canvas probes

[jkyberneees]

What

• conversation() reuses the joined convPrefix (and its cached line count) when the finalized-message count is unchanged — the hot path for the ~12 Hz render flush while a turn streams. Per-block invalidation drops the joined prefix too, since it embeds the changed block.
• paintCanvas caches its SGR probe colors per theme; theme switch invalidates.
• resize skips the glamour renderer rebuild and full-transcript re-render when the wrap width is unchanged (height-only / no-op resize bursts), backfilling never-rendered finalized messages once.
• Adds benchmarks for conversation, streaming refresh, token-batch ingestion, delta accumulation, paintCanvas, and clampLines — the repo previously had none.
• Fix from adversarial review: jumpToAgentStep now invalidates the step/message caches it mutates.

Measured (M2 Pro)

path	before	after
conversation(), 800 msgs steady-state	14.3 ms	8.1 µs
refresh() streaming, 201 msgs	5.2 ms	0.57 ms

Verification

• 3 adversarial reviewers on the branch diff; one MEDIUM stale-cache finding fixed pre-PR
• make fmt / make vet / make lint (0 issues) / go test -race -count=1 ./... all green