Paint the full retained transcript instead of a smaller window

[TheGreatAxios]

Summary

• The stream log is already capped at a fixed number of retained rows with an absolute base index threaded through every touch point, so a second, smaller paint window on top of that cap was redundant — painting every retained row is what makes all of it reachable by scrolling
• The paint tree rebuilds directly from the retained log (no separate windowing pass or collapse marker); appending a row adds one node and removes only what the retention trim evicted; replacing a row in place (the streaming-token path) still resolves to a single-node retext once eviction has started, since the row-children helper now excludes the eviction notice by node identity rather than array length
• Evicted rows get a notice above the oldest retained one, updated in place, so the boundary reads as dropped rather than as the true start of history
• Deleted the old windowing helpers and their dedicated test file — nothing calls them once the log itself is capped

Verification

• bun run typecheck and bun run build clean
• bun run test (the package's bun test ./src ./tests ./evals): 3946 pass / 0 fail across 303 files, two consecutive runs
• src/tui-opentui/transcript-long-log-scroll.test.ts: scrolling to the top of a 650-row session reaches row 50 (oldest retained), not row 49 (evicted), with the "50 earlier rows dropped" notice present; appending past the cap evicts at most one painted node; the eviction notice retexts in place across repeated evictions; replaceStreamRowAt called once after eviction has started removes at most one painted node (was 601 before this fix — the eviction notice's presence made the row-children length check permanently look like a broken mapping, forcing a full rebuild on every streamed token past the cap)

Closes CL-5553

[linear-code]

CL-5553