Get the markdown stack and the PNG exporter out of the initial chunk

[dovvnloading]

Problem

check-bundle-size.mjs has carried the same note through four amendments:
the ADR-019 budget for the initial chunk is 500 KiB (512,000 bytes), and we
were ~52% over it at 781,060. Every amendment said closing the gap needed
work "no stage owns".

So I measured it rather than guessing — a sourcemap attribution of the initial
chunk, charging every output byte back to the module it came from:

	bytes	share
react-dom	132,810	17.0%
app:lib/bridge-core	84,345	10.8%
@xyflow/react	83,000	10.6%
app:app/canvas	81,900	10.5%
app:app/chrome	77,841	10.0%
@xyflow/system	46,877	6.0%
micromark-core-commonmark	27,571	3.5%
…
all node_modules	511,393	65.5%
all app source	264,981	33.9%

Dependencies alone were 607 bytes under the entire budget, before a line of
app code. But two of them had no business being there at all.

Change

The Document View's markdown machinery

App.tsx statically imported DocumentViewPanel, which pulled in
DocumentViewMarkdown (react-markdown + six remark/rehype plugins +
react-medium-image-zoom) and documentViewHeadings — which runs its own
unified/remark-parse pass so the table of contents' ids provably match what
rehype-slug renders.

The eleven node views that also render markdown were already lazy. This one
eager importer was holding the whole unified/micromark/mdast stack in the
initial chunk for every session, including the many that never open the panel.

html-to-image

Reached from AppBar's Export PNG button and the command palette's export
command. Both now import it on activation.

The shell stays eager — that's the design

The panel is an <aside> that animates width 220ms ease from 0, and a CSS
transition does not fire on a freshly mounted element. Lazy-mounting the shell
would have cost the first open its slide-in. Only its contents moved.

A latch, not an isOpen check, gates the two dynamic imports: React.lazy
inside an always-rendered subtree requests its chunk moments after page load —
splitting the bundle without deferring the fetch, which is the whole point.
hasOpened latches on first open and never resets, so closing still doesn't
unmount: scroll position, search text and width survive exactly as before. Same
reasoning, and the same latch, as App.tsx's own LazySurface.

One observable difference

extractHeadings now resolves through a dynamic import, so it is state fed by
an effect rather than a useMemo, and the outline is empty for one microtask
after content changes. DocumentViewToc already renders nothing below two
headings, so that tick looks like a document with no outline.

Result

	before	after
Largest chunk	781,060	595,517 (−185,543, −23.8%)
Dependencies in it	511,393	328,585
Over the ADR-019 budget by	~52%	~16%
Chunks fetched on first paint	—	2

Both ceilings re-anchored down, and the comment now records what is actually
left instead of deferring it again: of 595,517 bytes, ~328,600 is dependencies
(React ~140,000, React Flow and its d3 deps ~175,000 — neither splittable) and
~262,300 is app source, concentrated in lib/bridge-core, app/canvas and
app/chrome. The remaining gap is app-source splitting, not a dependency
decision.

On the tests

The panel test asserting "content stays mounted when closed" was correct to
fail — it rendered a never-opened panel. I replaced it with two that state the
real contract rather than weakening it: a never-opened panel does not load its
markdown chunk, and a panel that has been opened keeps its content mounted
after closing.

commands.test.ts's export assertion carries a comment recording that it was
once vacuous (expect(() => run()).not.toThrow() held for run: () => {}). It
now awaits the microtask rather than losing that — it still fails if the command
is wired to nothing.

Test plan

• npm run check — schema, typecheck, lint, 2,195 vitest tests, build,
  bundle gate: all green.
• Python suite: 3,285 passed, 20 skipped.
• Verified in the running app against the built bundle: the <aside> is in the
  DOM before anything opens, at width 0 with width in its transition-property
  list; exactly 2 JS chunks are fetched on first paint; and the
  exportCanvasPng chunk appears only after clicking Export PNG.

🤖 Generated with Claude Code