Keep transcript headings stable while the paragraph below them streams

[TheGreatAxios]

Summary

• Renders a streaming markdown row's body as two stacked renderers once a heading has closed: a "frozen" one holding everything through that heading, and a "live" one holding the still-streaming tail
• Most rows have no closed heading yet, so they keep painting through a single renderer, same as before this fix — the split only allocates a second renderer once a heading actually closes
• The frozen half is non-streaming and is never handed new content while the tail keeps growing, so it is never asked to re-highlight once written
• The boundary is derived from the row's own text, tracking fence state (both fence characters, matching-or-longer closers only, a closing line may carry no trailing text per CommonMark) so a # line inside a fenced code block — a shell or Python comment, for instance — is never read as a heading. Also recognizes an ATX heading indented up to 3 spaces, per CommonMark.

On using the library's own parser instead of a hand-rolled scan: checked this directly before writing the regex. MarkdownRenderable's block/token state (_parseState, _blockStates) is underscore-prefixed and not part of its declared public API, and the module that builds it (renderables/markdown-parser.js, which exports parseMarkdownIncremental) has no entry in the package's exports map — it cannot be imported through any supported path, only by reaching into an internal module the package does not expose. No usable public surface exists, so the boundary is derived locally instead.

Verification

• bun run typecheck, bun run build, and bun test all exit 0 (4339 pass, 0 fail)
• src/tui-opentui/markdown-rows.test.ts adds:
  • a mid-stream test that samples the heading's painted span after every delta with no settle wait (the settle-then-assert pattern the rest of the file uses cannot see a transient re-highlight). Against the unfixed renderer this test fails, reproducing the shake directly: the heading's span reverts to literal "### Title" on a delta. Against this fix it stays stable across every delta.
  • an object-identity test confirming the frozen renderer is the same instance, with unchanged content, before and after the tail grows
  • a row with no closed heading still paints through a single renderer (no wasted allocation)
  • fence/heading edge cases: a # comment inside a fence (with and without a heading before it), unmatched fence lengths and characters (4 not closed by 3, 3 closed by 4, backtick/tilde never cross-close), a closing-fence-shaped line carrying trailing text (does not close, per CommonMark), 3-space fence indent recognized and 4-space not, an unclosed fence at end of input, and an indented heading
  • paragraph-directly-before-list spacing and ten-item ordered-list marker width, both unchanged from the pre-fix renderer

Layout parity with the pre-fix renderer is verified for headings, lists (including a list directly under a heading or a paragraph with no blank line, and ten-item ordered lists), paragraphs, and tables.

Closes CL-5559

[linear-code]

CL-5559