fix(sync): rename misleading field, add network/sync tests, enable coverage

[VelarisW58]

Fixes issue #51 and improves test infrastructure:

• Renames TargetReceipt.rollback_snapshot to rollback_baseline_hash — the old name implied an actual rollback snapshot id/dir, but the value was always just the current config's content hash (issue sync.ts: dry-run receipt's rollback_snapshot field is mislabeled #51)
• Adds explicit network-failure tests for registry.ts: fetch rejection, non-ok HTTP response, both fallback URLs failing, and MCPM_DEBUG warning logging (5 new tests, using vi.resetModules/vi.doMock to isolate each scenario from the real on-disk cache)
• Adds sync.test.ts covering readRC/writeRC/addToRC (10 tests) — previously untested
• Enables @vitest/coverage-v8, wired into vitest.config.ts and a new npm run test:coverage script

Verified: tsc --noEmit, vitest run (70/70), eslint, npm run test:coverage (produces a real v8 report).