Comment on: v0.4+ candidate: live-fork via memfd-backed source RAM + uffd_wp (deferred from v0.3)

Surface RFC posted at #174 — `DESIGN-v0.4-USER-API.md`. Pins the CLI / REST / SDK shape before implementation starts so we don't re-litigate `--live` vs `--mode live` vs `--wp` mid-coding. Companion to the existing kernel-mechanism doc (`DESIGN-v0.4.md`), use-case inventory (`DESIGN-v0.4-USE-CASES.md`), and FC integration spike (`DESIGN-v0.4-PHASE3-SPIKE.md`). Five open questions tagged for review in the doc; the most consequential is fail-vs-silent-fallback when the kernel doesn't support `uffd_wp` on memfd. Feedback welcome here or on the PR.