Two T10-flavoured changes in one commit, each independently ship-able
on its own:
1. `web_surface_frame::rgba_hash` switches from std's
`DefaultHasher` (SipHash13, ~1.5 GB/s) to `ahash::AHasher`
(~10 GB/s). At 1080p (8 MB per frame) the dedup key drops from
~5 ms to ~0.8 ms per cache-miss frame, returning roughly 25 % of
the 16 ms scroll budget that was being spent hashing the
newly-arrived RGBA payload. ahash was already in the dependency
graph transitively via hashbrown, so this only adds a direct
`ahash = "0.8"` line and one Cargo.lock entry.
2. `docs/t10-iosurface-plan.md` records the full architectural
roadmap for the actual zero-copy path that supersedes the
software-pipe pipeline: `OffscreenRenderingContext` against a
hardware surfman adapter, IOSurface-backed surface on macOS,
mach-port handoff to the GPUI process, MTLTexture import as an
external sampler. The document explains why each currently
shipped commit (`840255f`, `a80d039`, `e02c0fd`, `7f3b8b4`, plus
this hash swap) is a stepping stone that eventually deletes
itself once the IOSurface path lands, and names the upstream
API gap in `servo-paint-api` that blocks step 2.
cargo test --bin ely_app: 120 passed, 0 failed, 2 ignored.
Hash collision probability remains ~1 in 2^64; AHash uses the same
keyspace as the previous SipHash13.
