Phase 8 B: real Ed25519 keygen/signing, verified against OpenSSL

Extends the previously verify-only ed25519.c with ed25519_keygen() and
ed25519_sign() per RFC 8032 5.1.5/5.1.6, reusing every point-arithmetic
primitive verify already had -- only seed expansion/clamping and
per-message nonce derivation are new. Signing is deterministic; only
keygen ever touches entropy, via a caller-supplied seed (virtio_rng,
Phase A) -- keygen still generates nothing itself.

New scalar_muladd() (scalar25519.c) for signing's S = (k*a + r) mod L,
the one scalar op verify never needed. Schoolbook multiply into a u128
wide accumulator with one final carry pass -- deliberately the same
shape as fe25519.c's existing multiply, which has a documented history
of a real bug from carrying mid-accumulation instead of in one pass.

Verified against an independent implementation, not self-consistency:
a throwaway host harness against Python's cryptography library (OpenSSL-
backed) across 6 trials (5 random seed/message pairs + the empty-message
case) produced byte-for-byte identical pubkeys and signatures every
time. Clean compile on all three architectures and a full 3-arch QEMU
acceptance boot, conservation intact, no panics or guest errors.

Nothing calls the new functions from the live kernel path yet -- that's
Phase C (the MINT word itself), still open, documented in FABRIC-3.md.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01U14ET9CWAtbQMbYqomKgXd

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