Kernel K1: move anim, save, autosave into kernel/ (pure code motion)

[nyblnet]

First of four PRs extracting the shared kernel ahead of the Spaces/Dash
fan-out (per docs/PLATFORM.md §9's kernel/app boundary). This one is pure
code motion — no behavior change, proven by byte analysis.

What moved

• kernel/src/{anim,save,autosave}.ts (git mv — history follows)
• kernel/src/doc.ts: KernelDoc — the envelope (docId, title), the ONLY
  doc fields kernel modules may read; save/autosave retyped to it (type-only)
• docContentKey stays slides-side (in the autosave facade): "what counts as
  content" is a per-app model question

Design choices (from the planning audit)

• NOT an npm workspace. The moved modules have zero npm dependencies, so a
  workspace buys nothing yet — while a root-lockfile move would break three CI
  steps and every existing worktree's node_modules symlink. kernel/ is plain
  TS source with its own tsconfig; convert to a workspace later in one sweep
  if a shared dep ever appears.
• Facades at the old paths (slides/src/anim.ts etc. re-export kernel):
  zero consumer-file churn, window.bento untouched by construction, and
  relative-.ts imports work identically in vite, tsc (bundler resolution),
  and Node type-stripping — the repo's existing pattern (test-sync.ts).
• Dev-only server.fs.allow: ['..'] so vite dev serves ../kernel.
• CI gains a standalone kernel typecheck step.

Verification

• Byte identity: built shell vs shipped v1.0.8 — everything outside the
  bento-rt runtime blob is byte-identical (incl. the #bento-doc block);
  the runtime is length-identical (1,153,874 bytes both) with exactly 6
  identifier-rename-only diff regions (minifier short-name reassignment from
  docContentKey's declaration moving). No structural change.
• Typecheck: kernel standalone + slides tsc -b, both clean.
• Splice conformance gate passes; CRDT rig SEEDS=100 ALL PASS (sync untouched:
  git diff main -- slides/src/sync is empty).
• Dev-server smoke: boots through the facades, edits work, kernel autosave
  writes recovery snapshots to IndexedDB, Object.keys(window.bento)
  unchanged, zero console errors.

Next up: K2 (update/i18n/charts with the configureApp/registerI18n seams),
K3 (compress-script parameterization), K4 (spaces/ scaffold proving the seam).