Implement verified Triple DES ECB on ARMv7

[reaperhulk]

Adds complete baseline ARMv7 Triple DES ECB support for the existing in-place Rust API, with 16-byte and 24-byte keys and both directions. Depends on #511; this PR contains only the ARMv7 implementation and its API/test/benchmark architecture gates.

All five entrypoints are verified against the merged shared spec: key expansion, block encryption/decryption, and ECB encryption/decryption. Assembly is generated by Lean. No Spec or TCB changes. The scalar implementation keeps the two 32-bit Feistel halves in registers, shares IP/FP across all three DES passes, and uses Boolean S-box circuits with fixed scratch locations. The public ECB counter stays in a register preserved by the verified block primitive; ECB saves its link register outside block scratch.

Reviewed OpenSSL DES and AWS-LC DES. Both use secret-indexed round/key tables. This implementation uses their three-pass composition and register halves with constant-time Boolean circuits and fixed key permutations. The shared benchmark covers both key lengths and directions alongside OpenSSL at 64, 1024 and 16384 bytes; native ARMv7 measurements come from CI.

Validation: full Lean build (5,861 jobs), emitter generation and check including standard-axiom/compiler audits; all six repository check scripts; root and benchmark formatting/clippy; ARMv7 cross-clippy; the entire Rust suite under QEMU with Wycheproof, including all 500 published NIST ECB cases in both directions, block-aligned splits, parity and invalid lengths; full native baseline-feature Rust tests. After rebasing onto current main, targeted artifact/dependency builds and the emitter check pass. The Rust API and tests are shared with #504, whose combined CI coverage passed at 100% before the rebase.