feat(update): bare uffs --update updates end-to-end (ordinary-user default)

[githubrobbi]

The surgical phases (snapshot/acquire/apply/doctor/recover) are a power-user toolkit. The ordinary user types uffs --update and expects it to update — so now it does, and only when needed.

uffs --update (no action)

1. Detects the install + reads the latest release — one non-mutating fetch via a new lean uffs-update check helper subcommand (latest=<tag>, no download).
2. If a newer release is available or the install is version-skewed → runs the full journaled acquire + apply (auto-rollback on failure).
3. Otherwise prints ✓ UFFS is already up to date (vX.Y.Z) and touches nothing — crucially it will not stop/restart services when already current.

The old detect-only view → uffs --update check

Non-mutating "is an update available?":

• ✓ Up to date (vX) / ⬆ Update available: vX — run \uffs --update` to install`. Offline → a clear notice, no error.

snapshot / acquire / apply / doctor / recover are unchanged.

Implementation

• uffs-update: check subcommand (resolves latest via the existing github::fetch_release, prints latest=<tag>).
• uffs-cli: acquire::latest_version() spawns it; assess() → UpdatePlan::{UpToDate,Available,Offline} drives bare-update vs check; normalize_tag strips a leading v for comparison (unit-tested).
• Help, module docs, and cli-grammar.md §5 updated (none = update; + check).

Verification

• Full pre-push gate green; host + Windows-MSVC clippy clean; 129 uffs-cli/uffs-update tests pass.
• Smoke: check → "Up to date (v0.6.5)"; bare uffs --update → no-op when current (no service restart).