feat: hardware_aes() to detect the silent software-AES fallback; stay on aes 0.8

[coderdan]

Refs #86. This replaces the approach in #106 (now a draft).

Why not aes 0.9

aes 0.9 removes the --cfg aes_armv8 flag, but on aarch64 it is 2–2.5x slower per AES call than 0.8.4 with the flag. On an Apple M4 that makes:

• chained encryption about 3x slower
• full encryption 32–52% slower
• comparisons 59–123% slower

The measurements are in docs/benchmarks/2026-10-12-aes-0.9-results.md: an A/B/A run with under 1.2% drift between the two A runs, plus a microbenchmark of the AES calls alone. The upstream x86 regression (RustCrypto/block-ciphers#574) was fixed in 0.9.2, but aarch64 is still slow in 0.9.3, and no open issue reports it. aws-lc-rs was also measured and isn't faster.

What this PR does instead

It keeps aes 0.8 and makes the silent fallback that #86 worried about detectable:

• ore_rs::hardware_aes() (new public fn): returns true only when this build uses hardware AES on the current CPU. On aarch64 that means cfg(aes_armv8) plus CPU support; on x86 it means AES-NI. Applications can assert it at startup or in a test.
  • Why not a build-script warning: Cargo only shows cargo:warning for path dependencies, so a warning from ore-rs would never reach crates.io users.
• tests/hardware_aes.rs: fails on an aarch64 host that has AES when the flag is missing, so the aarch64 CI job catches a RUSTFLAGS override. Verified locally both ways:
  • passes with the workspace config
  • with RUSTFLAGS=-Dwarnings, fails with: "this CPU has AES, but ore-rs was built without --cfg aes_armv8; is RUSTFLAGS overriding .cargo/config.toml?"
• README: documents the RUSTFLAGS pitfall and hardware_aes(), and links the benchmark write-up.
• Cargo.toml: adds cfg(aes_armv8) to check-cfg.

Verification

Rust 1.78 (CI's toolchain on main):

• all tests pass, including the doctest
• clippy -D warnings and fmt --check are clean
• the new doc comment adds no rustdoc warnings. The four that remain are fixed by docs: fix intra-doc links to private items; gate rustdoc warnings in CI #107.

clippy -D warnings is also clean on 1.89.