Swap SipHash13 → AHash for the live-frame dedup key + record T10 plan

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.

crates/ely_app/Cargo.toml

@@ -9,6 +9,7 @@ rust-version.workspace = true
 live-site-smoke = []
 
 [dependencies]
+ahash = "0.8"
 directories.workspace = true
 ed25519-dalek.workspace = true
 ely_browser_core = { path = "../ely_browser_core" }