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

include/starkernel/scalar25519.h

@@ -26,4 +26,12 @@ void scalar_reduce512(uint8_t out[32], const uint8_t in[64]);
  * non-malleable signature component per RFC 8032), else 0. */
 int scalar_lt_L(const uint8_t s[32]);
 
+/* out = (a*b + c) mod L, all 32-byte little-endian scalars (a, b, c need
+ * not already be reduced mod L, though every caller in this codebase
+ * passes already-reduced inputs). Needed for EdDSA signing's
+ * S = (k*a + r) mod L step -- verify never needed scalar multiplication,
+ * only reduction, so this didn't exist until signing did. */
+void scalar_muladd(uint8_t out[32], const uint8_t a[32], const uint8_t b[32],
+                    const uint8_t c[32]);
+
 #endif /* SCALAR25519_H */