build: document opt-level=z + CGU=1 miscompile and CGU>=2 alternative

[ch4r10t33r]

Summary

Docs-only follow-up to #759. Expand the [profile.openvm-release] / [profile.risc0-release] comments in rust/Cargo.toml with the narrowed root cause from a bisection session on x86_64 Linux (rustc 1.95, AMD EPYC-Genoa + GitHub runners) and cross-reference the upstream issue.

Key findings now captured in-file:

• The miscompile requires the combination opt-level = "z" AND codegen-units = 1. Either change alone clears it.
• Faulting code is lean_vm::tables::utils::eval_virtual_bus_column once inlined into lean_prover::prove_execution — a stack reload of a spilled AVX2 lane inside a 16-YMM-register-live basic block.
• The comment now lists the three bisected combinations so a future reader can immediately see the alternative (opt-level = "z" + codegen-units >= 2) if they ever want to revisit z for binary-size reasons.
• Added a cross-reference to the upstream issue leanEthereum/leanMultisig#198 alongside the existing risc0: prove_execution crashes with GPE in xmss_aggregate during CI #734 reference.

No code/config values changed — profile keys and values are byte-identical to main. Diff is 14 lines of comment text.

Test plan

• cargo fmt --manifest-path rust/Cargo.toml --all -- --check clean
• cargo clippy --manifest-path rust/Cargo.toml --workspace -- -D warnings clean
• cargo metadata --manifest-path rust/Cargo.toml --no-deps parses without error
• Local zig fmt --check pkgs build.zig clean
• CI green (docs-only change; expect no regression from build, multisig-glue: bump prover profile opt-level from z to s (#734) #759's shipped workaround)