v1.7.4 — Native user-level CA trust stores

Janus now makes browsers trust its local CA natively on every
platform, strictly at the user level: no sudo, no UAC, no system
CA bundles, no crawling through per-profile cert9.db databases.

WHAT CHANGED
- TrustStore trait with an idempotent ensure() built over the
  injected ProcessRunner (phase-3 micro-object architecture,
  scripted-fake testable).
- Windows: CurrentUser\Root via native certutil — Chrome, Edge
  and Yandex pick the CA up instantly.
- macOS: login Keychain via native security add-trusted-cert —
  Chrome, Edge and Safari.
- Linux: Chromium NSS shared user DB via certutil -t "C,," —
  supports both the M146+ default (~/.local/share/pki/nssdb) and
  the legacy ~/.pki/nssdb, creates an empty database when none
  exists, and degrades quietly when certutil is absent.
- Config.auto_trust (default true): trust is ensured fire-and-
  forget at startup, check-first, so native platform confirmations
  appear at most once per profile.
- Firefox: security.enterprise_roots.enabled = true on Windows and
  macOS (the official Mozilla mechanism); manual import stays the
  documented path on Linux, where the flag is not supported.
- docs/dev-rules/CA-TRUST.md codifies the storage and trust
  boundaries; the Firefox guide is split per operating system.

QUALITY
- 9 angry unit tests for the NSS backend plus cfg-gated suites for
  Windows and macOS: one assertion per test, scripted process
  fakes, no OS calls; full suite green (99 lib tests), cargo
  check --tests clean.
- Built on the yegor256-test-campaign baseline (phases 1-3,
  70.67% line coverage).