Milestone 6: mkcapsule signing + capsule_birth.c wiring, WARN-only

First attempt shelled out to `openssl pkeyutl -sign` (fork/execlp, not
system() -- avoided shell string interpolation of the key path).
Corrected on request: no new external host binary dependency when the
repo's own code can do the job -- same standing preference as the
earlier anti-file correction. Rewritten to link ed25519_sign() (already
verified against OpenSSL in Phase B) directly into mkcapsule.

New tools/pkcs8_ed25519.c: a narrow DER walker (same shape as
x509_ed25519.c, deliberately not shared -- small enough that
duplicating a few TLV-walking lines beat threading a header between the
kernel crypto tree and host tooling) extracting the raw seed from the
intermediate's PKCS#8 private key, plus a minimal self-written base64
decoder (PEM is openssl genpkey's default output; no decoder existed
anywhere in the repo). Verified end-to-end before wiring anything in:
the extracted seed's derived pubkey matches the cert's exactly, and a
full self-contained sign+verify round-trip (zero openssl) passes.

CapsuleDesc had no spare bytes, so signatures live in a new parallel
CapsuleSigEntry array, emitted by a new `mkcapsule --sign-key <path>`
flag (omitted/missing key -> has_sig=0 everywhere, graceful, not a
build failure -- CI has no access to the offline key).

New capsule_sig.c/.h: capsule_verify_signature(), a separate function,
not folded into the already-tested capsule_validate(). Finds and caches
the embedded intermediate cert's pubkey once per boot, then verifies
against it. Wired into all three capsule_validate() call sites in
capsule_birth.c via log_message(LOG_WARN, ...) -- never refuses yet,
per the earlier staged-rollout decision.

Verified independently, both directions, live in the real kernel: a
full clean build (38 signed capsules) boots clean on all three
architectures with zero warnings. Separately, hand-corrupted one byte
of Mama's own init.4th capsule's stored signature (not its payload/hash,
which capsule_validate() already catches and would have masked the
test) and rebuilt just the changed object: produced exactly "capsule
sig: init.4th: INVALID -- signature does not verify" on boot, and the
kernel still reached ok> -- proving warn-only doesn't refuse anything
yet. Reverted before the final, untampered 3-arch acceptance pass.

Still open: flipping WARN to hard-refuse (separate, deliberate step)
and the BLOCK_MAP.md signature-status column. 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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