bench(streaming): mpsc vs crossbeam vs direct callback (refs #482) (#485)

Add `streaming_channels` criterion bench measuring the per-event cost
of the producer/consumer hand-off the FFI streaming surface pays for
every FPSS event delivered to a buffered consumer. Five variants:

- `std_mpsc_unbounded` — current live shape (`std::sync::mpsc::channel()`
  + `recv_timeout(100ms)` poll).
- `crossbeam_bounded_{256,1024,8192}` — lock-free SPSC candidate at
  three capacity points covering the proposed backpressure range.
- `direct_callback` — no channel; producer invokes an `extern "C" fn`
  through a `Box<dyn Fn>` trampoline, modelling the C/C++ tier-1 path.

Payload mirrors `ffi::streaming::FfiBufferedEvent` field-for-field:
`#[repr(C)]` tagged event with `TdxContract` embedded in every data
variant plus two heap-owned tails (`Option<CString>` + `Option<Vec<u8>>`).
Sizes match the generated `fpss_event_structs.rs` byte-for-byte
(Event = 448 B, BufferedEvent = 488 B on x86_64).

`crossbeam-channel` is added to dev-dependencies only so the runtime
dep graph is unchanged. The implementation switch ships in a follow-up
PR after we read the numbers.

Author: userFRM

Cargo.lock

@@ -118,6 +118,12 @@ zeroize = { version = "1.8.2", features = ["derive"] }
 
 [dev-dependencies]
 criterion = { version = "0.8.2", features = ["html_reports"] }
+# Bench-only: lock-free SPSC/MPSC channel candidate for replacing
+# `std::sync::mpsc` on the FFI streaming hot path (issue #482). Kept in
+# dev-dependencies so the runtime dep graph is unchanged until the
+# implementation switch ships in a follow-up PR after we read the
+# numbers from `benches/streaming_channels.rs`.
+crossbeam-channel = "0.5.15"
 # Test-only: load captured-response fixtures and their sidecar .meta.toml
 # files for the decoder-regression suite (`tests/test_decode_captures.rs`).
 # `toml` is already a build- and opt-dependency here; adding it to
@@ -154,6 +160,10 @@ harness = false
 name = "bench_fpss_event"
 harness = false
 
+[[bench]]
+name = "streaming_channels"
+harness = false
+
 [[bin]]
 name = "generate_sdk_surfaces"
 path = "src/bin/generate_sdk_surfaces.rs"