fix: read a table's own caption on HTML import

[dereuromark]

The bug

<table><caption> is how HTML captions a table, and it is what pandoc emits for every captioned table. The importer dropped it silently:

htmlToCarve('<table><caption>Fruit prices</caption><thead><tr><th>A</th></tr></thead>…</table>')
// value:       "|=A|\n| 1 |\n"        <- caption gone
// diagnostics: []                     <- nothing said

The returned AST node came back as {type, rows} with no caption field.

table() builds its rows by walking for tr elements, so a <caption> child is simply never visited.

Why this was never a representation problem

The slot already existed on every side:

• resources/ast-schema.json defines table.caption
• the parser fills it from ^ text after the rows
• carveToHtml renders it back to <caption>

So the shape was always spellable — the element was just unread.

The second case it exposed

A figure-wrapped table can arrive with two captions: its own <caption> and the figure's <figcaption>. Carve spells one ^ line per host, so emitting both produced:

| 1 |
^ Inner
^ Outer     <- re-reads as <p>^ Outer</p>

That's worse than the silent drop. The figure wrapper has no Carve spelling at all (markup-carve/carve#1211 documents exactly this), so the figure's caption is the one that cannot survive. The table keeps its own, and the figcaption is reported as table-degraded — the code the contract already defines as "a table could not be represented structurally", which avoids adding a code the spec's html-import-schema.json would have to enumerate first.

Behavior after

Input	Carve	Diagnostics
<table><caption>Fruit prices</caption>…	|=A| / | 1 | / ^ Fruit prices	none
<figure><table>…<figcaption>Outer	| 1 | / ^ Outer	none
<figure><table><caption>Inner…<figcaption>Outer	| 1 | / ^ Inner	table-degraded

The middle row is unchanged — a figure-wrapped table with no caption of its own still puts the figcaption in the slot the table left empty, losing the wrapper element but no text.

Cross-engine

Measured on the same input:

Engine	Before
carve-php	already reads it (findFirstDirectChildByTagName($node, 'caption'))
carve-js	dropped it — fixed here
carve-rs	same gap: html_import.rs hardcodes caption: None

carve-rs needs the equivalent fix; filing separately.

Verification

Full suite: 383 files, 9484 tests, 0 failures. npm run typecheck clean. Three regression tests added covering the table's own caption, the figure-wrapped table with an empty slot, and the two-caption conflict (including that the output contains no leaked <p>^).