Add host-owned SourceCode and Diff rendering for the app and plugins

[ymichael]

What was wrong

BB rendered code in two independent places and had no way for a plugin to change either. The file preview drove @pierre/diffs' File view and the diff card drove its FileDiff view, each assembling its own options record, its own line-selection wiring, and its own code-theme lookup. A plugin that wanted to render source or a diff had a third path: import @pierre/diffs directly through the runtime shim and rebuild the host's behavior by hand. plugins/github did exactly that — synthesizing a diff --git header for GitHub's REST patches, calling parsePatchFiles, and running two MutationObservers (one on the root class, one on data-bb-code-theme-*) to keep Pierre's theme in step with BB's.

That is three copies of one capability, and it made "change how bb renders code" impossible to express: there was nothing to replace.

What changed

One host boundary per capability. SourceCodeHost and DiffHost (apps/app/src/components/code/) resolve an exclusive plugin replacement through the same resolveReplacement + PluginReplacementSlot path the sidebar thread list and file openers already use, and otherwise render BB's own renderer. Both BB renderers (BbSourceCode, BbDiff) sit behind lazy(), so a replacement that never delegates never downloads them and experimental_Original costs nothing until it is rendered.

Public API (all experimental_, with entries in docs/api_to_audit.md):

experimental_SourceCode: { content, path, overflow?, highlightedLines?, className? }
experimental_Diff:       { patch, path, view?, overflow?, showLineNumbers?, className? }

app.slots.experimental_sourceCodeRenderer({ id, title, description?, component })
app.slots.experimental_diffRenderer({ id, title, description?, component })

A replacement receives fully resolved semantic props plus a bound experimental_Original, so it can delegate per call without re-entering resolution. Host-only inputs — the pre-parsed ParsedGitDiffFile, the raw patch text, the highlighter cache key, selection-to-composer — never cross the boundary, and no @pierre/diffs, Shiki, FileOptions, or ParsedGitDiffFile type appears in the public contract. experimental_Diff owns patch normalization, so a patch with no diff --git header (GitHub REST, a bare @@ hunk) renders without the caller synthesizing one; content that parses to no hunks degrades to plain monospace text instead of an empty diff.

Migrations. The native file preview, timeline file diffs, and the environment diff panel's file bodies all render through the boundary, so one registration covers BB's surfaces and plugin surfaces alike. plugins/github renders experimental_Diff and drops its @pierre/diffs devDependency along with both MutationObservers; its bundle no longer references the Pierre runtime shim (417.0 KB → 409.0 KB). The shim itself stays for compatibility with existing plugins.

User control. Settings → Appearance gains Source code and Diffs rows beside Sidebar — Automatic / bb (built-in) / each registered provider, per client, each row hidden when no plugin supplies that renderer. A renderer takes over surfaces there is otherwise no route back from, so this is what keeps "installing activates it" reversible. lib/plugin-replacement-preference.ts now owns that automatic/built-in/named-provider rule and the thread list consumes it too, retiring its private copy and the resolveThreadListReplacement wrapper only its own test still called. Registered renderers also appear in the plugin detail's capability list.

Two cleanups fell out. The opaque diffViewOptions: Record<string, string | boolean | number> threaded through five components became the semantic DiffPresentation (view / overflow / showLineNumbers). The renderers own light/dark selection themselves, which made the themeType prop chain from ThreadTimelineSurface down to TimelineFileDiffBlock dead; it and ThreadTimelineTheme are removed.

Docs and generated artifacts. docs/api_to_audit.md gains three entries (components, slots, experimental_Original); the bb-plugin-authoring skill and the bb guide plugins chapter now point at the host components rather than a direct @pierre/diffs import, and the skill's own coverage test is extended so the new slots and their props stay documented. Bundled SDK declarations, the runtime export manifest, and the templates bundle were regenerated with the repository scripts. No wire changes, so no HOST_DAEMON_PROTOCOL_VERSION bump.

How you verified

Regression coverage for what the boundary actually promises — each of these fails without the corresponding behavior:

• a replacement that never delegates leaves BB's renderer chunk unloaded; delegating through experimental_Original loads it;
• the replacement receives resolved semantic props only — no parsed file, no cache key, no selection-to-chat;
• with no patch text in hand the host reconstructs a single-file patch that re-parses to the same file;
• crash and no-registration paths land on BB's renderer;
• BB's diff renderer follows applyResolvedCodeTheme live — the behavior plugins/github previously got from a DOM MutationObserver;
• DiffFileCard, a first-party surface, renders its text body through the same boundary;
• a built-in pin renders BB's renderer with the plugin still installed and enabled, and an explicit pin survives a later plugin whose id sorts ahead of it (where automatic selection would silently swap the user's renderer).

Commands run:

• pnpm exec turbo run typecheck --filter=@bb/app --filter=@get-bb/plugin-sdk --filter=@bb/plugin-build --filter=@bb/templates --filter=@bb/server --filter=bb-plugin-github --filter=@bb/cli — passed.
• pnpm exec turbo run test --filter=@bb/app — 367 files / 2909 tests passed.
• pnpm exec turbo run test --filter=@bb/server — 182 files / 1730 tests passed.
• pnpm exec turbo run test --filter=@get-bb/plugin-sdk --filter=@bb/plugin-build --filter=@bb/templates --filter=bb-plugin-github --filter=@bb/cli — 631 tests passed.
• pnpm exec turbo run lint --filter=@bb/app — 0 errors (the repository's existing 147 react-compiler warnings; several are carried over verbatim with the extracted renderer code).
• pnpm exec turbo run build --filter=bb-plugin-github — builds; the bundle contains experimental_Diff and no Pierre shim reference.

Bundle impact. Boot payload 1655.6 → 1659.1 KB raw and 448.8 → 453.7 KB brotli against the 1671.0 / 456.4 budget; 40 → 43 chunks. I confirmed this is not new code on the boot path — stubbing out the patch reconstruction moved it by 0.3 KB — it is the extra chunks compressing slightly worse than fewer larger ones. All forbiddenBootPackages still pass. New lazy chunks: BbDiff 2.6 KB raw / 1.1 KB brotli, BbSourceCode 6.1 KB raw / 2.4 KB brotli. Brotli headroom drops from ~7.6 KB to ~2.7 KB, which is worth knowing before the next feature lands on that budget.

Deliberately not migrated. Markdown fenced code blocks are genuine source but run sugar-high — a synchronous ~2 KB highlighter with no worker pool, no shadow DOM, and no async settle, which is what a streaming chat timeline needs. Routing them through the host would either regress that or require a second per-surface default renderer, contradicting one host per capability. Raw-log surfaces (EventCodeBlock, terminal output) are monospace styling, not code rendering, and are untouched.

Open questions for review, all recorded in docs/api_to_audit.md: whether a per-client pin is the right scope for something as visible as every diff (and whether users expect one combined choice rather than two); whether a plugin should be able to change how another plugin's experimental_Diff renders; and whether the silent crash fallback is right where the thread list toasts.

AGENT GENERATED: by Claude Opus 5