Keep the open preview editor mounted across offset-shifting remote CRDT changes

[cscheid]

Fixes #420 (braid bd-84ljmbaf).

Problem

With the rich-text editor open in q2 preview --allow-edit (or hub-client), any remote CRDT change that shifts the active block's byte offsets — i.e. any length-changing edit anywhere earlier in the document — disturbed the editor:

• a focusout/focusin blip (~30 ms) that drops keystrokes typed in the window,
• caret reset to end-of-block, undo history lost,
• on the rich surface, silent loss of the uncommitted draft (it reseeds from the AST; the textarea reseeds from the preserved editDraftRef and only blips).

Root cause

The first render after an external re-render still carried the stale editTarget.anchorR0. The dispatcher's anchor match (editTarget.anchorR0 === resolved.sourceEntry.r[0]) then failed for every block, unmounting the editor for one frame; the P2.3b self-heal useLayoutEffect re-anchored one render too late and mounted a fresh editor. The re-anchor logic (findReanchorCandidate, content-first and symmetric) was fine — only its timing was wrong.

Fix

PreviewRoot now derives the re-anchored target during render (effectiveEditTarget, a useMemo running findReanchorCandidate over the freshly-parsed pool/content) and feeds it to the context and editTargetRef. The first post-change render already matches the shifted offsets, so React reconciles the mounted editor in place — same DOM node, focus never leaves, caret/selection/doc untouched. The self-heal effect still persists the re-anchor into state and still owns the DROP path (a remote edit to the active block itself closes the editor, unchanged).

Testing

• New editor-survives-remote-shift.integration.test.tsx (real PreviewRoot, real pointer-event activation) pins element identity (toBe) and document.activeElement retention across forward and backward shifts, for the rich surface and the textarea (the textarea test also pins the dirty draft). Written first per TDD: 3 tests failed on the unfixed tree exactly on the remount assertions; all 4 pass with the fix.
• preview-renderer suites: 578 unit + 632 integration passing. Full cargo xtask verify (including hub-client build:all + test:ci): green.
• End-to-end through the real binary: q2 preview --allow-edit in Chrome — typed a draft into the rich editor, applied two offset-shifting edits to the file on disk (title grow, then shrink); the ProseMirror DOM instance survived both, zero focus events, caret and draft intact, and a subsequent Cmd-Enter commit landed on the correct paragraph with the remote edits preserved.

Out of scope (follow-up strands filed)

• bd-vrdglm6m — a whole-block insert/delete above the active block still remounts (index-based key={i} shifts).
• bd-yu5uj8am — merge remote content into the open editor instead of the self-heal DROP when the active block is edited remotely.

Plan/diagnosis: claude-notes/plans/2026-08-26-gh420-editor-focus-crdt.md.

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