[alex]

Part of #991. Stacked on #2606.

This is the first registration to use Artifact.written (#2568). On AArch64, vg_x25519_base writes only the first 1024 bytes of its 8 KiB scratch. Its artifact now says so with written := [("scratch", 1024)], and the emitter exports VG_X25519_BASE_SCRATCH_WRITTEN. x25519::base() then wipes just those bytes, where it used to wipe all 8 KiB. That saves about 80 ns of vg_zeroize per call on N2, about 1% of x25519_base.

Other targets still wipe the whole buffer.

Changes

• Proof
  • x25519Base_correct (Proof/X25519/AArch64/Base/Main.lean) gains a ScratchTail conjunct: when out and scratch are disjoint, no byte of scratch at offset 1024..8191 changes.
  • It is proven from the Outside frames of the comb, the inversion and the final stores, through outside_tail.
  • Those frames' bounds in the base and comb engines are tightened from 7368 to 968. They are the bytes past the working area, which the proofs never used.
  • x25519Base_written (Base/Verified.lean) turns this into Sig.writtenWithin from the contract's precondition, using Exec.det.
• Registration: written and writtenOk only; no tactics.
• Rust: on aarch64, base() wipes VG_X25519_BASE_SCRATCH_WRITTEN bytes. Every other target keeps size_of::<[u64; 1024]>(). Each cfg arm runs on its own platform, so coverage is unchanged.

No changes to TCB/ or Spec/.

Testing

• The touched modules and the X25519 artifact build. Emit.lean --check passes, with the audits.
• check_lean_imports, check_lean_modules, check_registrations, check_lean_speed, check_variants, check_arch_gates and check_mcdt all pass.
• cargo fmt --check and clippy pass on x86-64 and aarch64.
• The X25519 Wycheproof and unit tests pass on x86-64, and on aarch64 under qemu.

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