[runtime/iouring/waker] add loom model

[andresilva]

This PR adds loom coverage for the runtime io_uring waker, which coordinates producer-side publishes and out-of-band wakes with loop-side eventfd/futex parking through a packed atomic state word. The tests model the production sequence and wait-bit state with loom atomics, replace kernel wake surfaces with loom-visible futex and eventfd models, and use a minimal queued-request model so missing release/acquire synchronization around request publication can be caught.

The models cover the main wakeup paths: publish publishing to pending, wake publishing to clear_wait, armed publish/wake races through the eventfd path, fully-idle futex parking, eventfd wake deduplication, sticky unarmed wakes, final-handle drop wakeups, sequence wraparound, and repeated loop drain cycles. They also include deterministic generated producer programs for producer-only, eventfd-loop, and futex-loop histories.

The goal is to make the waker's concurrency contract explicit: every published submission must be observed exactly once, pure wakes must not advance the submission sequence, wait targets must be cleared after wakeup, unarmed wakes must not queue eventfd readiness, eventfd readiness must be coalesced and acknowledged correctly, and queue writes made before publish/wake must be visible after the loop observes pending or resumes through clear_wait.

Depends on #3692.

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Benchmark results

Tip

✅ PASSED: No benchmark exceeded the regression threshold.

Benchmark comparison table

Benchmark	Baseline (main)	Current	Delta	Threshold	Status
qmdb::merkleize/variant=any::unordered::fixed::mmr keys=10000 ch=false sync=false	1.587 ms	1.575 ms	-0.74%	10.00%	✅ PASS

Baseline commit(s): 99825a0a1ac8

[patrick-ogrady]

I’m tempted to suggest putting the real impl in a single block before the loom impl rather than interleaving but won’t block on it.

[codecov]

Codecov Report

❌ Patch coverage is 98.11321% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.82%. Comparing base (c32f535) to head (db06010).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
runtime/src/iouring/waker.rs	98.11%	0 Missing and 1 partial ⚠️

@@           Coverage Diff           @@
##             main    #3711   +/-   ##
=======================================
  Coverage   95.82%   95.82%           
=======================================
  Files         458      458           
  Lines      180034   180073   +39     
  Branches     4208     4209    +1     
=======================================
+ Hits       172525   172563   +38     
+ Misses       6168     6167    -1     
- Partials     1341     1343    +2     

Files with missing lines	Coverage Δ
runtime/src/iouring/waker.rs	96.62% <98.11%> (+0.08%)	⬆️

... and 4 files with indirect coverage changes

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