feat(mpmc): add capacity reservation to bounded senders

[jiengup]

Summary

Closes #297.

• Add BoundedSender::reserve, try_reserve, and a borrowed mpmc::Permit so callers can wait for capacity before constructing a message, as requested in #297.
• Add integration coverage for exact capacity, FIFO grants, cancellation, permit release, and receiver disconnection. Update the API documentation and changelog.
• Commits since upstream/main: c42f549 (contract tests) and 28a0b14 (implementation).

Design Notes

Queued messages, held permits, and grants to waiting senders share the configured capacity. Sends and reservations receive grants in wait-queue order. Cancelling a granted operation or dropping an unused permit passes its slot to the next waiter. A permit reserves capacity, not message order, and Permit::send returns the value if receivers have disconnected.

Benchmarks

Lower is better. Results compare upstream/main (0f46831) with this branch (28a0b14). Each value is the median of 10 Divan run medians on an Apple M3 Pro with Rust 1.98.0. Microbenchmarks use capacity 1; batch benchmarks use capacity 64 and transfer 16,384 messages per iteration.

Existing bounded MPMC operation	upstream/main	This branch	Time change
try_send → try_recv	11.47 ns	12.12 ns	+5.7%
send → recv	15.56 ns	16.73 ns	+7.5%
Wake blocked send	33.83 ns	38.39 ns	+13.5%
Threads, producers→consumers 1→1	1,178.5 µs	1,164.5 µs	−1.2%
Threads 1→8	16,050 µs	15,505 µs	−3.4%
Threads 8→1	15,505 µs	14,355 µs	−7.4%
Threads 8→8	5,959.5 µs	6,359 µs	+6.7%
Tokio, current thread 1→1	268.7 µs	278.5 µs	+3.6%
Tokio, current thread 1→8	336.2 µs	339.2 µs	+0.9%
Tokio, current thread 8→1	301.3 µs	330.5 µs	+9.7%
Tokio, current thread 8→8	307.2 µs	320.0 µs	+4.2%
Tokio, 4 workers 1→1	385.0 µs	395.0 µs	+2.6%
Tokio, 4 workers 1→8	1,099.5 µs	1,187.5 µs	+8.0%
Tokio, 4 workers 8→1	1,095.5 µs	1,262.0 µs	+15.2%
Tokio, 4 workers 8→8	1,318.0 µs	1,363.0 µs	+3.4%

New reservation operation	This branch
try_reserve → Permit::send → try_recv	16.52 ns
reserve → Permit::send → recv	21.56 ns
try_reserve → drop permit	10.17 ns
reserve → drop permit	12.35 ns
try_reserve on a full queue	4.64 ns
Wake blocked reserve, then send	31.40 ns

Existing single-thread round trips are 5.7%–13.5% slower. Batch results vary by topology and include both improvements and regressions; the largest measured regression is 15.2% for Tokio with 4 workers and 8 producers→1 consumer. Reservation-specific cases were measured in a temporary benchmark copy and are not part of this PR.

Validation

• cargo x test passed, including the bounded MPMC reservation tests and doctests.