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.

Cargo.toml

@@ -1,5 +1,5 @@
 [package]
-name = "alknet-vault"
+name = "alkvault"
 version = "0.1.0"
 edition = "2021"
 license = "MIT OR Apache-2.0"
@@ -7,7 +7,7 @@ description = "Local key vault: BIP39 mnemonic generation, SLIP-0010 Ed25519 HD
 repository = "https://git.alk.dev/alkdev/alkvault"
 
 [lib]
-name = "alknet_vault"
+name = "alkvault"
 
 [features]
 default = []