…AAS crates

Tier 0 (payload-scale):
- SigningContext: Added Borrowed(&[u8]) variant, factory uses from_slice() eliminating full payload copy
- MST JWKS cache: Arc<JwksDocument> wrapping, get() returns refcount bump not 5-50KB deep clone
- MST proof blobs: extract_proof_blobs() returns Vec<ArcSlice> instead of Vec<Vec<u8>>

Tier 1 (per-validation hot paths):
- Certificates: 23 fact struct fields String -> Arc<str>, ParsedCert fields -> Arc<str>
- MST proofs: Hash fields [u8;32] fixed arrays, path Vec<(bool,[u8;32])>
- CWT claims: claims_bytes Vec<u8> -> ArcSlice

Tier 2 (per-operation):
- AAS digest: Hash digests stay on stack as GenericArray, eliminated 4x heap alloc per signature
- AAS validation: eku_oids uses .to_string() for Vec<String> compatibility

All 7,886 tests pass. Clippy clean. Zero regressions.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…cates static key usage strings, CWT Arc<str> claims, MST fact Arc<str>/Cow, DID Cow<str> policies, factory ArcStr fields, CounterSignature Cow details, validation message_arc() zero-copy

- AKV: Inline digest computation, eliminate hash_sig_structure method and 3x .to_vec()
- Certificates: Key usage Vec<String> -> Vec<&'static str> for 10 static strings
- CWT: Fact fields String -> Arc<str>, use cose_sign1_message_arc() for zero-copy
- MST: sha256_hex->Arc<str>, coverage->&'static str, details->Option<Arc<str>>
- DID: DidX509Policy::Eku uses Cow::Borrowed for OID string literals
- Factory: Hash envelope contributor fields -> ArcStr, stack GenericArray digests
- Validation: CounterSignatureEnvelopeIntegrityFact.details -> Cow<'static, str>
- All 7,886 tests pass, clippy clean

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…on pack READMEs

- SigningError/FactoryError: String tuple variants -> named Cow<'static, str> struct
  variants for zero-alloc static messages and self-documenting field names
- PayloadTooLargeForEmbedding: positional (u64, u64) -> named { actual, max }
- FFI: Remove misleading #[repr(C)] from 6 validation structs embedding Rust types
  (Vec, Arc, Option) — these are opaque behind *mut pointers, never passed by value
- Add #[must_use] to 6 builder/options types: CoseSign1Builder, SigningOptions,
  DirectSignatureOptions, IndirectSignatureOptions, TrustPolicyBuilder,
  TrustDecisionAuditBuilder
- Add comprehensive README.md for certificates, MST, and AKV extension packs
- All 7,886 tests pass, clippy clean, rustfmt clean

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…rror types

- HeaderError: All String variants -> Cow<'static, str> (39 call sites in cwt_claims.rs)
- ReceiptVerifyError: All 7 String variants -> Cow<'static, str> (38 call sites)
- ValidationFailure: message/error_code/property_name/attempted_value/exception -> Cow
- CoseSign1ValidationError: String variants -> Cow<'static, str>
- AKV: key_type/curve_name fields -> Cow<'static, str> with static lookups
- AKV: Service metadata literals use Cow::Borrowed
- Validation FFI: .into_owned() at boundary for FFI String requirement
- All 7,886 tests pass, clippy clean, rustfmt clean

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Documents the DID:x509 method implementation including parsing, building,
validation, and resolution capabilities. Covers architecture, all modules,
key types with usage examples, supported policies, FFI surface, SCITT
compliance patterns, and memory design notes.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>