v1.19.0 "Afterburner"

Fifteenth release of the RustyNES-parity roadmap: an optional PGO/BOLT pipeline for the
shipping rustysnes binary, deliberately last per the plan (after mobile-specific hot-path
work landed, so the profile isn't invalidated).

Added

• PGO/BOLT pipeline (Mobile-track-adjacent, deliberately last per the RustyNES-parity
  roadmap): scripts/pgo/run.sh instruments, trains against the committed permissive ROM corpus
  (via a new crates/rustysnes-test-harness/src/bin/pgo_trainer.rs binary — the gilyon
  CPU-instruction suite plus a handful of undisbeliever HDMA-glitch/INIDISP-hammer ROMs, chosen
  for control-flow breadth beyond the single steady-state headless_frame bench ROM), and
  rebuilds the shipping rustysnes binary with the merged profile. New
  .github/workflows/pgo.yml: workflow_dispatch + release-tag push only (never the PR gate —
  an instrument+train+rebuild cycle is far too slow for that). Promotion requires both a
  measured >3% Criterion speedup over the plain release build and a byte-identical re-run
  of the full --features test-roms oracle under the PGO-merged profile, citing
  docs/adr/0004's determinism contract — never promotes on speed alone. An optional Linux-only
  BOLT post-link stage chains onto an already-promoted PGO binary, best-effort.
• Fixed a real, latent CI gap found while building this: rust-toolchain.toml didn't list
  llvm-tools-preview, and dtolnay/rust-toolchain silently ignores the rust-setup composite
  action's own components: input whenever a rust-toolchain.toml file exists in the repo (the
  same behavior already found and fixed for ios.yml in v1.16.0) — without this, cargo-pgo's
  .profraw/.profdata merging would have silently failed to find the component on a fresh CI
  runner. Added llvm-tools-preview directly to rust-toolchain.toml, the actual effective
  source of truth.
• Verified for real in this development environment: the full instrument → train (5 committed
  ROMs) → optimized-rebuild pipeline produces a genuine, running rustysnes binary, and the
  determinism oracle (cargo pgo optimize test) passes cleanly under the PGO-merged profile. The
  local A/B speedup did not clear the >3% promotion bar on a short local training run (as
  documented honestly in docs/performance.md — this is an expected, not a failure, state; a
  short/narrow local run isn't representative of CI's real 3600-frame training sweep).

Fixed

• A real bug in pgo.yml's BOLT stage, found in PR review: re-invoking the whole
  scripts/pgo/run.sh between cargo pgo bolt build and cargo pgo bolt optimize ran a
  separate, non-BOLT PGO cycle that never fed BOLT's profile data and could clobber the
  bolt-instrumented binary with an unrelated plain-PGO one. Fixed per cargo-pgo's own
  documented BOLT+PGO combined workflow: --with-pgo on both cargo pgo bolt build and
  cargo pgo bolt optimize, with the erroneous run.sh re-invocation removed. Real BOLT profile
  gathering (running the actual GUI frontend binary against a workload) stays out of scope —
  this project's frontend has no headless CLI mode — so the fix deliberately uses cargo-pgo's
  own documented profile-less BOLT fallback instead.