Rebrand alknet-vault -> alkvault

Rename the crate from alknet-vault to alkvault across source and docs:
- Cargo.toml: package name and lib name (alknet_vault -> alkvault)
- src/ doc comments and doc-test use statements
- tests/ use statements and one string literal

Convert references to non-vault alknet ADRs (003, 005, 008, 010, 014,
064) that were broken local links into @alkdev/alknet: cross-repo
references, matching the alktype sibling pattern. Local ADR/OQ
references are now proper links. Rewrote monorepo path references
(crates/alknet-vault/src/...) to the flat layout (src/...). Fixed
sdd_process.md package name (@alkdev/storage -> @alkdev/alkvault).

ADR/OQ renumbering is deferred to a subsequent pass per the alknet-
origin numbering convention. Generic prose 'vault' and type names
(VaultServiceHandle, VaultServiceError, etc.) are unchanged.

Build, 108 tests, and clippy all pass clean.

docs/architecture/decisions/021-key-rotation-via-version-indexed-paths.md

@@ -244,9 +244,10 @@ are expected.
 
 ## References
 
-- ADR-020: HD derivation for encryption keys (this ADR builds on the
-  version-indexed path scheme)
-- OQ-22: Key rotation mechanism (resolved by this ADR)
+- [ADR-020](020-hd-derivation-for-encryption-keys.md) — HD derivation for
+  encryption keys (this ADR builds on the version-indexed path scheme)
+- [OQ-22](../questions/022-key-rotation-mechanism.md) — key rotation
+  mechanism (resolved by this ADR)
 - [encryption.md](../encryption.md) — AES-256-GCM, EncryptedData
 - [service.md](../service.md) — encrypt, decrypt, rotate methods
 - [mnemonic-derivation.md](../mnemonic-derivation.md) —