S4: homefs is a 28-line import-satisfying stub — needs design doc then CoW implementation

Stub-burndown register S4 (P1 design doc, P2 implementation). File: kernel/src/fs/homefs.home.

First deliverable: docs/design/homefs.md — CoW model, extent layout, snapshot trees — written BEFORE implementation ([MASTER_PLAN §11.2]).

Blocks: Phase 2 storage; Phase 6 snapshot-rollback; factory reset; pantry snapshot-guarded updates.

Design doc landed: docs/design/homefs.md (e48e7eb) — object-ID layout, commit protocol, snapshot/rollback mechanics, and a gated 7-step implementation plan. S4 stays open until steps 4–5 pass executed tests per register rule 1.