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/service.md

@@ -376,10 +376,10 @@ don't miss them.
 
 ## References
 
-- Implementation: `crates/alknet-vault/src/service.rs`,
-  `crates/alknet-vault/src/cache.rs`
-- Tests: `crates/alknet-vault/tests/service_tests.rs`,
-  `crates/alknet-vault/src/service.rs` (unit tests),
-  `crates/alknet-vault/src/cache.rs` (unit tests)
+- Implementation: `src/service.rs`,
+  `src/cache.rs`
+- Tests: `tests/service_tests.rs`,
+  `src/service.rs` (unit tests),
+  `src/cache.rs` (unit tests)
 - [protocol.md](protocol.md) — `DerivedKey` and `KeyType`
 - [encryption.md](encryption.md) — `encrypt` / `decrypt` cryptographic details