test(e2e): close the rest of the pre-migration coverage gaps

[christianhuening]

Completes the baseline started in #15. Those were unit tests over a real bound editor; these are the four gaps that genuinely need the stack — they cross the API, the database, the Rust serializer, or a second browser session.

No production code changes. 3 new e2e files, 2 modified.

comment-anchors.spec.ts — new, and the important one

comments.spec.ts posts every comment straight to the API with position_y: null and never clicks the floating button, so nothing exercised anchoring at all.

The failure it guards is silent by construction: when the ySync binding can't be reached, the encoder returns null and the caller persists an empty string. The comment saves, the sidebar lists it, only the highlight is missing.

Asserts the stored anchor is non-empty, a highlight renders over the right words, it survives a reload (decoding against a binding that didn't encode it), and it follows the text when a peer inserts ahead of it — which a byte offset fails and a relative position passes.

Verified by stubbing getMapping() to null: all three fail, while comments.spec and comments-realtime stay green.

editor-markdown-roundtrip.spec.ts — new

Only paragraph, image and excalidraw_board ever travelled editor → Y.Doc → to_markdown. Drift is punished asymmetrically: an unknown node makes the export 500, but an unknown mark hits to_markdown's catch-all arm and vanishes without a trace.

Authors every canonical node and mark by pasting Markdown — so the nodes are built by the editor's own schema, not the server's parser — then asserts each construct survives the export, by name.

Verified by renaming the underline mark: underline did not survive the round trip, while import-markdown and collab stay green.

This also covers the link-title fix from #16 end to end: the export contains [link](https://example.test "Link title").

two-users-converge — Alice can now actually see Bob

Its docText() helper deliberately strips caret labels, so a caret that stopped rendering left every assertion in the file green. Now asserts the caret exists, carries the peer's name, and has a real colour — the awareness payload is validated against a 6-digit hex pattern.

Verified by forcing colorFor to "transparent".

editor-toolbar — aria-pressed tracks the caret

The existing test only asserted on document contents, so a toolbar frozen at its initial state would have passed. Now checks that state follows the selection rather than the click, and that the isActive("table")-gated controls appear and disappear.

(While writing this I had an assertion passing on a typo — toolbar-add-row instead of toolbar-table-add-row. Fixed, and it now exercises the gate in both directions.)

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docText() moves to support/ since two files need it.

Local run: 60 passed. Three failures, all environmental and unrelated: ws-reconnect needs toxiproxy (network_mode: host doesn't work on Docker Desktop), oidc needs Dex env this run didn't set, and tree-reorder's "nest" is the known macOS-only failure that passes in CI.

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