chore: design cleanup, dead-code purge, README/doc refresh

Audit-driven follow-up after the SAFETY-debt commit (Oracle steps 6-7).
End state: cargo clippy --release --all-targets still 0 errors; private_interfaces
and type_complexity warnings cleared.

Design cleanups (Oracle step 6):
  - cap_portal.rs: introduce PortalFormatInfo struct to replace the
    Rc<Cell<Option<(u32,u32,u32,u64)>>> cross-callback hand-off. Self-
    documenting struct fields replace positional tuple access at the
    format-change and process callbacks.
  - avhw.rs: import_dma_buf_to_vaapi signature collapses from 8 args
    (fd/width/height/drm_format/modifier/stride/offset) to
    (*mut AVBufferRef, &PwDmaBufFrame). Callers in avhw.rs,
    state_portal.rs, and vaapi_import_bench.rs now pass the frame by
    reference instead of unpacking 7 fields just to repack them. Drops
    the unused width parameter and the too_many_arguments(8/7) warning.
  - state.rs: visibility hygiene. EncConstructionStage and WlrHeadInfo
    downgrade pub -> pub(crate); State.stage field downgrades to
    pub(crate). These are internal state-machine types not exposed
    across the crate boundary; making them pub(crate) clears all
    private_interfaces warnings without leaking more types.

Dead-code purge (Oracle step 7):
  - transform.rs: remove unused Rect struct, transform_basis,
    screen_to_frame, fit_inside_bounds helpers and their 18 dedicated
    tests. Transform enum and transpose_if_transform_transposed remain
    (both are actively used by state.rs and avhw.rs). File shrinks
    from 409 -> 109 lines.

Repository housekeeping (Oracle step 7):
  - .gitignore: add review.json (stray review-tool output that
    regenerates per run).
  - README.md: refresh CLI table to match src/args.rs (now lists
    --backend, --no-persist, --port-as-WebRTC-signaling, --max-bitrate,
    --stats). Add capture-backend explainer + 4 new usage examples.
    Note in README points readers at src/args.rs as the authoritative
    source. Remove stale 'WebTransport, unused in MVP' description.

avhw.rs: AsRawFd import annotated with a rustc-quirk explanation — the
import triggers a false 'unused_imports' warning but E0599 if removed.
Left as-is with explanatory comment rather than chasing the lint.

All 79 remaining unit tests + 3 integration tests still pass. Cargo
build --release clean.

src/bin/vaapi_import_bench.rs

@@ -417,17 +417,10 @@ fn import_frame(
 ) -> Result<ff::frame::Video> {
     // SAFETY: frames_ctx is a live VAAPI frames context configured for the capture format; frame
     // carries a valid DMA-BUF fd and metadata from PipeWire for the duration of the call.
+    // SAFETY: frames_ctx is a valid VAAPI frames context; `frame` carries the
+    // DMA-BUF metadata read by the function.
     unsafe {
-        import_dma_buf_to_vaapi(
-            frames_ctx.as_ptr(),
-            frame.fd.as_raw_fd(),
-            frame.width,
-            frame.height,
-            frame.format,
-            frame.modifier,
-            frame.stride,
-            frame.offset,
-        )
+        import_dma_buf_to_vaapi(frames_ctx.as_ptr(), frame)
     }
 }
 
@@ -940,20 +933,11 @@ fn main() -> Result<()> {
 
     // SAFETY: delegates to avhw::import_dma_buf_to_vaapi (itself an unsafe fn).
     // frames_ctx is a valid AVBufferRef from AvHwFrameCtx::for_capture above;
-    // first_frame's fd/width/height/format/modifier/stride/offset are all sourced
-    // from the PipeWire-formatted PwDmaBufFrame. See that function's own SAFETY
-    // contract for the full rationale.
+    // `first_frame` is the PipeWire-formatted PwDmaBufFrame whose metadata the
+    // function reads directly. See that function's own SAFETY contract for the
+    // full rationale.
     let vaapi_frame = unsafe {
-        import_dma_buf_to_vaapi(
-            frames_ctx.as_ptr(),
-            first_frame.fd.as_raw_fd(),
-            first_frame.width,
-            first_frame.height,
-            first_frame.format,
-            first_frame.modifier,
-            first_frame.stride,
-            first_frame.offset,
-        )
+        import_dma_buf_to_vaapi(frames_ctx.as_ptr(), &first_frame)
     };
 
     match &vaapi_frame {