Spec: add key revocation, key distribution, size limits, replay enfor…

[jaschadub]

…cement

Defense-in-depth additions (no wire-format change).

• §4.1 model_hash now constrained to "sha256:" + hex of weights, omitted otherwise (no more signature-binding theatre on an undefined field).
• §4.3 (new) Size limits: pin JSON ≤ 64 KiB, extra ≤ 32 entries, key ≤ 128 B, value ≤ 1 KiB, vec_dim ≤ 2^20, sig exactly 64 B.
• §5 step 0 makes size-limit enforcement a verifier MUST before parsing; step 7 requires verifiers to MUST-check vectorpin.{record,collection, tenant}_id against caller-supplied expected values.
• §5 adds KEY_EXPIRED, RECORD_MISMATCH, COLLECTION_MISMATCH, TENANT_MISMATCH, PARSE_ERROR to the required failure-mode taxonomy.
• §7 split into rotation and revocation; introduces (valid_from, valid_until) registry semantics so a compromised key invalidates post-compromise pins without invalidating historical ones.
• §10 (new) Key distribution: fingerprint format, transparency-log SHOULD, TOFU NOT RECOMMENDED, per-tenant kid separation SHOULD.
• §9 security considerations updated to reference §4.3 DoS limits, §7 revocation window, and §5 step 7 replay enforcement.
• §11 versioning adds an explicit downgrade-resistance argument.

Wire format unchanged; existing pins continue to verify.