Add per-node attribution to q2-debug AST view

[shikokuchuo]

Closes #115.

Summary

Adds per-node authorship to the q2-debug AST view. Each node is coloured by the author who last edited the source bytes behind it, with a hover badge showing the author and a relative timestamp. Off by default — toggle "Authorship" in the Settings sidebar (q2-debug only).

How Automerge attribution meets the AST

Every AST node carries a sourceInfoId (the s field) that points into the parse's sourceInfoPool. SourceInfoReconstructor (from @quarto/annotated-qmd) resolves that id to a byte range { fileId, start, end } in the original source.

Separately, an attribution producer replays Automerge history and records, per character, who wrote it and when. Producers expose only a minimal query interface:

interface AttributionSource {
  queryByteRange(fileId, byteStart, byteEnd): { actor, time } | null;
}

Attributing a node is then: resolve its byte range via the reconstructor, hand that range to the source, and (for multi-byte ranges) take the most recent writer — "last touch wins". The interface is byte-indexed (UTF-8, as emitted by WASM); the Automerge-backed source keeps an internal byte→char map since Automerge text is UTF-16.

Cold start vs incremental updates

useAttribution(filePath, sourceText) owns the build lifecycle:

• Cold start: buildRunListAttribution replays full Automerge history, 500 entries per chunk, yielding via requestIdleCallback before every chunk. The hook returns null while the build is pending, so the document paints un-attributed first.
• Incremental: on each sourceText change (debounced 500 ms), updateRunListAttribution applies only the new history entries to the existing runs synchronously. On HistoryCompactedError, we fall back to a full rebuild.
• Switching files aborts any in-flight build and clears state before the next cold start.

useNodeAttributionResolver(astContext, attributionCtx) then wraps the source + reconstructor into a memoised, cached getNodeAttribution(sid) → { actor, time, color, name } | null. color and name come from an identities map supplied by the Automerge sync layer; unmapped actors fall back to a short hex of the actor ID.

Rendering: text colour and hover tooltip

A node with a resolved attribution is wrapped in <span|div class="q2-attr-wrap" data-sid="…" style="color: <actor-color>">. The colour is plain inline style — no registry magic.

Hover is event-delegated: a single onMouseOver/onMouseOut pair on the container finds the nearest .q2-attr-wrap, reads data-sid, calls getNodeAttribution, and positions one floating <AttributionBadge> via getBoundingClientRect. One handler, one badge — not N of each.

Producer-swappable design (git-blame adapter)

AttributionSource is the only boundary that matters: anything above it (storage shape, producer) can change without touching anything below (hook, resolver, renderer).

The default producer is Automerge-backed and run-length encoded (attribution-runs.ts) — 4× faster updates, 5× faster queries, 20× smaller than the per-char prototype (attribution.ts) on realistic batched workloads.

attribution-gitblame.ts is a second, independent producer that parses git blame --porcelain, converts lines to byte-ranged runs using TextEncoder (so multi-byte UTF-8 lands on correct byte offsets), and returns an AttributionSource with binary-search queries. It touches no consumer code — concrete proof the boundary holds and Automerge is swappable.

Access from render components

Editor.tsx publishes AttributionContext ({ source, identities, sourceText }) to the render tree whenever attribution is on. Any renderer can opt in:

1. useContext(AttributionContext) to get the source.
2. useNodeAttributionResolver(ast.astContext, attributionCtx) for a getNodeAttribution(sid) function.
3. Read sid from a node's s field and look up { actor, time, color, name }.

ReactAstDebugRenderer is currently the only consumer; any other renderer picks this up in three lines.

Enabling the feature

A persisted attributionEnabled preference (default false) is exposed as an "Authorship" toggle in the Settings sidebar, shown only when the current format is q2-debug.

Editor.tsx passes filePath = null to useAttribution when the toggle is off, which short-circuits the history replay — zero cost when disabled, and flipping off mid-session aborts any in-flight build.

Other notes

• Minor type fixes in ts-packages/annotated-qmd so it compiles cleanly under hub-client's strict tsconfig.

[shikokuchuo]

After discussion with Carlos we're going to move this to a transform step of the render pipeline, so attribution will be available to all consumers - cli as well as hub-client.