fix(transcript): start reasoning blocks at boundaries

[clabby]

Summary

• key streamed reasoning by agent request and model-call index so concurrent runs cannot share a block
• coalesce reasoning only while its entry remains the transcript tail
• end reasoning continuity at assistant, tool, run, and real in-place transcript boundaries
• cover reused call indices, ordinary tool calls, resumed shell followups, concurrent runs, and ignored message updates

Testing

• cargo test -p tact tui::transcript::model::tests
• just check-fmt
• cargo check --all-features
• just clippy

Stack

Depends on #193.

[codspeed-hq]

Merging this PR will not alter performance

✅ 18 untouched benchmarks
⏩ 31 skipped benchmarks¹

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_{Comparing cl/reasoning-blocks (37ea12b) with cl/shell-kill-cleanup (d11219a)}

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. ↩