v1.6.0 "Lighthouse"

Second release of the RustyNES-parity roadmap.

Added

• Documentation site + PWA + accuracy ledger — a Material for MkDocs handbook (mkdocs.yml)
  is now published at https://doublegate.github.io/RustySNES/docs/, alongside the wasm demo
  (/) and rustdoc (/api/), replacing pages.yml with a combined web.yml that also enforces
  the existing <5MiB gzip wasm size budget (scripts/wasm_size_budget.sh) on every PR. The wasm
  demo gained PWA/offline support (manifest.webmanifest, a stale-while-revalidate sw.js
  service worker, a real icon.svg). New docs/accuracy-ledger.md maps every known
  approximation/divergence to an explicit disposition (Remediated / No-stricter-oracle-available /
  Deferred / Out-of-scope), the "why" companion to docs/STATUS.md's pass-count dashboard.
  docs/DOCUMENTATION_INDEX.md refreshed (was still stamped v0.4.0, referenced a nonexistent
  SALVAGE_MANIFEST.md).

Fixed (caught in PR review, #78)

• web.yml's cancel-in-progress was inverted relative to its own comment — PR size-budget
  checks could be cancelled mid-flight while stale main pushes weren't; corrected to cancel on
  push only.
• sw.js's fetch handler could resolve respondWith() to undefined on a truly offline first
  visit (network fails and nothing is cached yet); now falls back to a synthetic 503 response.