Health v2: Snapshot v2 (v2 signal in the snapshot format)

[belarusian]

The snapshot format gains a v2 shape; old v1 snapshots still load.

• fleet/health.py: ProjectHealth gains a defaulted health_v2: str | None = None field (last position, so existing keyword constructions stay valid).
• fleet/snapshot.py: save_snapshot(healths, path, git_states=None) computes each row's health_v2 via classify_health_v2(days, last_outcome, git_state) when git_states is provided; _health_to_dict/_health_from_dict round-trip health_v2 (an old v1 snapshot without the key loads with health_v2=None); _field_changes surfaces a health_v2 -> fragment (- for None) via _fmt_v2, appended after the v1 health fragment.
• fleet/cli.py: _cmd_snapshot passes git_states=_git_states(root) so a saved snapshot carries the v2 class.
• tests/test_snapshot.py: +7 v2 tests (round-trip, none-without-git, old-v1-loads, diff v2 change/unchanged/none-to-set, canary classes).
• tests/test_cli.py: snapshot-then-diff now surfaces health_v2 ->- transitions.

Gate: pytest 153 passed, ruff clean, mypy clean.

Closes #75 #76 #77 #78 #79