refactor(state): split 1598-LOC state.rs into directory + extract 13 Dispatch impls

Step 4a + 4b combined.

- src/state.rs (1594 LOC) -> src/state/mod.rs (struct + inherent methods +
  types + helpers; 999 LOC) + src/state/dispatch/ (13 Dispatch impls across
  6 files: registry.rs / wl_output.rs / dmabuf.rs / screencopy.rs /
  output_mgr.rs / buffer.rs).

Per Oracle audit: orphan rule permits Dispatch impls in submodules because
Dispatch is a foreign trait on local type State<S>. All State fields the
impls touch are already pub/pub(crate) — no visibility widening needed.

Verification (all green):
- cargo build / cargo build --release
- cargo test (79 lib + 3 integration = 82 pass, 1 ignored — unchanged)
- cargo clippy --all-targets -- -D warnings
- cargo fmt --check
- cargo check --bin vaapi_import_bench --bin sw_encode_bench

src/state/dispatch/mod.rs

@@ -0,0 +1,6 @@
+mod buffer;
+mod dmabuf;
+mod output_mgr;
+mod registry;
+mod screencopy;
+mod wl_output;