feat(native): host React block renderers in the segmented markdown views - #1027

…#1026)

* feat(core): collect replaceable blocks and share the streaming filter

Adds the C++ side of custom block renderers for EnrichedMarkdownText:

- BlockKinds: one table per replaceable kind (image paragraph, video block,
  link-only paragraph) with its node matcher, naming and streaming line rule.
  The collector, the annotator and the streaming filter iterate it, so a new
  kind is one entry here plus a margin entry per host.
- BlockSlots: collects slots and image occurrences in document order with
  stable ids (`<kind>:<fnv1a64 of url>:<n>`, slots and occurrences counted
  separately) and stamps the active ones on the AST in one walk. Nothing
  inside a list item is a slot, since the hosts lay lists out as text.
- StreamingFilter: the math, table, code fence and media rules iOS, Android
  and the JS collector all shared by copy now live once. Trailing image,
  link and video lines are withheld while their syntax is open (hidden mode
  waits for the next block), a bare URL stays withheld until the next line,
  and an inline image or link still being typed at the tail of the last
  paragraph is cut so it never flashes as text.
- The parser keeps link titles, so a link block can carry metadata.

Behaviour changes for existing streaming users: a trailing partial
`![alt](https://…` no longer renders as literal text, and `[label](url "t")`
links now carry their title in the AST.

Tests: block_slots_test (eligibility, id stability, annotation, the hash
fixture shared with the web collector) and streaming_filter_test.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

* chore(web): rebuild the wasm parser bundle

The parser now keeps link titles, so the committed bundle has to be rebuilt
for scripts/test-web-bundle.mjs to agree with the golden AST dump.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

* ci: run the pull request checks for PRs stacked on feat/block-renderers-*

The CI workflow only triggers for pull requests into main, so the upper PRs
of the block renderers stack (#1027 onto #1026's branch, #1028 onto #1027's)
got no checks. Temporary; remove once the stack has merged.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

* fix(core): read inline link syntax the way CommonMark does while streaming

Review findings on the streaming classifier, all with regression cases:

- a closed bracket followed by prose (`[INFO] done`) is prose, not an open
  link, so the line is no longer withheld;
- a link destination closes at its balanced `)`, so nested parentheses and
  titles with parentheses classify correctly in both the line classifier
  and the inline-tail scanner, which now share one reader;
- a closed autolink followed by text is prose, not a block;
- four or more spaces of indentation make the line indented code, which no
  classifier matches and the tail scanner leaves alone;
- an escaped bracket or bang is literal text;
- code spans are matched by backtick-run length, so a double-backtick span
  keeps its brackets.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

* fix(core): read nested labels and link titles while streaming

The inline syntax helpers now follow CommonMark further:

- A label closes at its balanced `]`, skipping escapes and code spans, so
  `[![thumb](u)](v)` and `[a [b]](u)` classify as link paragraphs and an
  outer link still typing around a closed image is cut, not shown as text.
- A destination may be `<pointy>` and may carry a quoted or parenthesised
  title; parentheses inside the title no longer count as delimiters, so
  `[a](u "t(")` is complete and `[a](u "t )` stays open.
- The tail scanner walks left over the `[` candidates until one is open.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

* fix(core): withhold a block line cut off inside its opening syntax

A stream that stopped at `<`, `<vid`, `<htt` or a lone `!` showed those
characters as text for a tick before the line became a video, link or
image block. A strict prefix of `<video`, `<http` or `![` on the trailing
line is now open syntax, like a half-typed URL.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

* perf(core): filter the stream in one pass and share one parse per document

The streaming filter split the document into copied lines up to three times
per tick, and the open-inline-syntax check rescanned the last line for every
`[`, which made a long single-line paragraph cost links x line length. It
now scans blocks once over string_view lines and makes one inline pass over
the trailing paragraph, so a label that wraps onto a second line is cut too.
A fuzz test checks every streamed prefix against md4c and a bench fails if
a pathological tail gets much slower than prose.

DocumentStore keeps the last filtered and parsed documents (16 entries, 8 MB)
so JS collect, the shadow node and the view share one filter and one parse
per tick. The parsed tree is never written to after the parse.

Block ids now hash kind, url, text and title, and image occurrences use their
own `img:` prefix, so a block id can no longer equal an occurrence id.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

* fix(core): start the trailing paragraph after blocks that end it

The inline-syntax pass walked back to the last blank line, so a stray `[`
in a heading, a setext heading or a quote above the trailing paragraph
withheld everything after it. The walk now stops at headings, underlines,
thematic breaks, deeper quotes and HTML blocks.

A `](` inside a destination already found to be prose no longer rescans
it, which kept `[x](` repeated linear. Both shapes are in the bench.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

---------

Co-authored-by: Claude Fable 5.1 <[email protected]>
Co-authored-by: Ernest <[email protected]>