[FEATURE]: Land two-stage TUI streaming freeze fix for #36043

[dcerisano]

• I have verified this feature I'm about to request hasn't been suggested before.

Description

Tracking issue for the fix plan — the freeze itself is reported in #36043,
this is not a re-report.

Diagnosis (from #41472): streaming pays O(n²) at two client-side stages.
Each PR applies and merges independently, in either order; together they
fully close the freeze:

• tui-delta-coalesce (fix(tui): coalesce message.part.delta store writes #48431) — coalesce message.part.delta store writes (V1)
• markdown-live-relex (fix(session-ui): grow markdown live tail in place #48432) — grow the live markdown tail in place

Related: #41472 (root-cause analysis; tui-delta-coalesce supersedes it).

Done = both merged, at which point I'll close this issue.