v1.5.0 "Bedrock"

First release of the RustyNES-parity roadmap: closes the gap between this project's own
feature/UX/accuracy maturity and its sibling NES emulator RustyNES, tracked in lockstep rather
than a frozen snapshot. This rung is CI safety net only — see to-dos/VERSION-PLAN.md's
"RustyNES-parity ladder" section for the full v1.5.0-v1.19.0 plan.

Added

• CI safety net — cargo test --workspace now runs on every PR/push to main (new
  test-light job), not only on a tagged release. A new changes/setup job pair computes a
  light-vs-full run mode per push (mirroring RustyNES's own pattern), and full-test/no_std/
  bench now also run on every push to main (previously tag-only), plus a weekly drift-net cron
  and manual dispatch. A new ci-success job is the one stable required-check name; branch
  protection on main now requires it. See docs/adr/0011.
• A shared .github/actions/rust-setup composite action factors the pinned toolchain version and
  cache-key convention out of ci.yml/pages.yml into one place.
• to-dos/LOCKSTEP-CHECKLIST.md — the process for re-checking RustyNES's own continuing
  development before scoping each subsequent rung in the parity ladder.

Fixed (caught in PR review, #76)

• The rust-setup composite action pinned dtolnay/rust-toolchain@master (a floating ref);
  changed to @1.96, matching what ci.yml's jobs already used before this release.
• The composite action's Linux frontend dependency list was missing libxkbcommon-x11-dev
  (present in CONTRIBUTING.md's documented list but never actually installed by the old inline
  per-job steps this action replaces) — added, and CONTRIBUTING.md reconciled to match exactly.