fix(tui): render local images reliably

[clabby]

Summary

• render local Markdown images inline only when the terminal supports a native image protocol
• degrade unsupported backends to ordinary clickable Markdown links
• accept absolute scratchpad and other local paths outside the workspace
• move metadata, decode, resize, and protocol preparation off the render thread
• bound image work and caches while preserving independent placement sizes
• hydrate restored transcripts without blocking first paint
• lazily retransmit images after focus and resume without changing their dimensions
• avoid redraw-time filesystem work and unnecessary image reconstruction

Testing

• cargo check --all-features
• just check-fmt
• just clippy
• just test (946 tests)

Stack

1. feat(prompt): link local images with Markdown #179 — Markdown image-link prompting
2. This PR
3. chore(release): prepare 0.6.6 #180 — prepare 0.6.6

Depends on #179. No matching open issue exists to close.

[codspeed-hq]

Merging this PR will not alter performance

✅ 18 untouched benchmarks
⏩ 31 skipped benchmarks¹

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_{Comparing cl/image-rendering (ab9ca94) with cl/image-cleanup (23c2078)}

Footnotes

1. 31 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩