ci: run fuzz targets in parallel, build CI test wheels without LTO

[brndnmtthws]

Two runner-time cuts borrowed from brndnmtthws/hessboost#124 (which itself took the ci profile and abi3 matrix from #221). Wall time is unchanged: the critical path is windows-latest / stable / default (218 s in main run 36335261574).

Changes

• Fuzz: the smoke step ran the 9 targets one after another (100 s of a 151 s job). It now runs one cargo fuzz run per CPU at a time; the build step already builds every target with the same flags, so each run only checks freshness. Logs go to $RUNNER_TEMP/fuzz-logs/. After all targets finish, each target's output is printed in its own ::group::, in list order: printing as each one ended interleaved the groups of targets finishing together. If any failed, the step exits 1 with an ::error:: naming them. Crash artifacts upload as before.
• Python: the python job's wheels are built only to be tested, but built with python/Cargo.toml's lto = "fat" and codegen-units = 1. The maturin step now sets CARGO_PROFILE_RELEASE_LTO=false and CARGO_PROFILE_RELEASE_CODEGEN_UNITS=16 (maturin-action forwards CARGO_* into the manylinux container). python-sdist and publish.yml still build and test with the real release profile. Wheel builds on main took 35–109 s (Windows 109 s).

Verification

• actionlint 1.7.12 (with shellcheck 0.11.0) on the workflow; shellcheck on the extracted fuzz step.
• Ran the extracted fuzz step locally on nightly with nproc shimmed to 4: all 9 targets ran their 11 s, step took 34 s (100 s sequential in CI).
• Failure path: a cargo shim failing fuzz-kem with exit 77. The other 8 targets still completed, fuzz-kem (exit 77) showed its output, and the step exited 1 with fuzz targets failed: fuzz-kem.
• cargo build --release --locked in python/ with the two overrides builds.
• CI timings on this PR will show the wheel build savings.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.32%. Comparing base (fbacd16) to head (f9bf5fb).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #222   +/-   ##
=======================================
  Coverage   68.32%   68.32%           
=======================================
  Files         115      115           
  Lines       10032    10032           
=======================================
  Hits         6854     6854           
  Misses       3178     3178           

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