fix(ui): 0.32.0 adoption feedback (default KaTeX loader, docs, HANDOFF in tarball)

[backnotprop]

Follow-up to the 0.32.0 adoption of @plannotator/ui by a host (Workspaces), which reported five mismatches. Additive only; Plannotator's own behavior is unchanged (its entries fill the math slot through math-eager before the first render, so the changed branch is never reached there). No server changes.

1. Default KaTeX loader isolated. The default import('katex') moves out of utils/math into its own module, utils/math-default-loader (loadDefaultMathRenderer), and utils/math now keeps the host loader as null until one is registered: the default is called only on the null branch, and a registered loader is never backfilled by it, not even after the host's load rejects (new unit test; the existing default path also gained a test since the split is what wires it back in). Honest limit, measured on a scratch Vite 6 consumer of this checkout: chunk emission is static, so a host build that registers a loader still emits the package's 484 KB katex chunk (never requested) unless it aliases math-default-loader at a stub. With that two-line alias the same build emits zero chunks carrying the KaTeX body; without a loader the chunk is emitted and requested as before. The alias recipe and the measurement are in HANDOFF.md "Lazy renderers and eager entries". tests/entry-assets.test.ts now pins the split (no import('katex') site in utils/math, the only one in utils/math-default-loader); the built-bundle registration and presence markers are unchanged.

2. onUnanchoredChange timing documented in README and HANDOFF: nothing is delivered for a document before the bridge's first post-restore report, per reload generation; a prop-side change before that point folds into that report, so a host must not gate on a prop-side delivery arriving first.

3. projectHostThreads documented as HTML-only: the projection pins blockId to "", offsets to 0, and carries no startMeta/endMeta. Markdown support was not added: the markdown Viewer restores by blockId plus text search inside that block, so a metas passthrough alone would never re-anchor a markdown thread; markdown threads still need the host's own projection.

4. maxAdditionalTargets documented: with the cap enforced upstream (bridge toggle, parent trust boundary on submit and restore, maxTargets on read), a host's cap-dropped handling is unreachable in normal operation and is a backstop only; byte-budget drops remain a separate, reachable path.

5. Packaging: HANDOFF.md joins the package files, so README's "See HANDOFF.md" references resolve in an installed copy. bun pm pack --dry-run: 362 to 363 files, unpacked 4.67 MB to 4.75 MB (the 86 KB HANDOFF plus the 1.2 KB loader module).

Verification: bun run typecheck clean; bun test packages/ui 729 pass, 516 skip (DOM-gated), 0 fail; DOM_TESTS=1 on MathBlock.firstPaint, InlineMarkdown and math tests 44 pass; build:review and build:hook succeed and tests/entry-assets.test.ts runs its built-bundle marker checks 24 pass, 0 fail; the scratch Vite consumer builds in all three variants (no loader, loader, loader plus alias).

AI-assisted (Claude) under maintainer direction.