feat(motoko): EncryptedMaps canister mixin for one-line canister integration

[marc0olo]

Draft — third of three PRs for #402 (DX: canister-integration helpers).

What

Uses Motoko's new actor mixins (moc 1.9.0) so the ic-vetkeys library can provide the entire EncryptedMaps canister interface, and refactors the reference canister to include it:

import EncryptedMapsCanister "mo:ic-vetkeys/encrypted_maps/Canister";

persistent actor class (keyName : Text) {
    include EncryptedMapsCanister(keyName, "my_domain_separator");
};

This replaces ~200 lines of hand-written delegation (one shared/query func per library method, ByteBuf↔Blob wrapping). It's the Motoko counterpart to the Rust macro in #404.

Why

• Interface correct by construction: the mixin is the single source of the endpoint set, so the Candid matches the @icp-sdk/vetkeys frontend exactly.
• Upgrade-safe: the mixin owns its stable state; included into a persistent actor the maps survive upgrades.

Verification (local)

• mops build compiles the mixin-based canister (moc 1.9.0 + local lib).
• All 18 existing canister integration tests pass against the mixin-generated Motoko wasm (make test — the repo's Rust integration suite run against the Motoko canister).
• The generated .did exposes the same method set as the canonical Rust .did.
• mops format clean.

Toolchain changes

• Bump moc to 1.9.0 (mixins) in the library + canister mops.toml.
• Canister icp.yaml recipe → @dfinity/motoko@v5.0.0, which delegates the build to mops (entry point + moc come from mops.toml).
• Use core's Array.map (the current library returns plain arrays, which have no .map method under moc 1.9.0).

⚠️ Draft / publish-bootstrapping notes

• Local dependency: to dogfood the unreleased mixin, the reference canister temporarily depends on the library via a local path (ic-vetkeys = "../../ic_vetkeys"). On release this flips to the published ic-vetkeys@0.6.0.
• mops.lock not committed for the canister: mops records a machine-specific absolute path for local deps, so committing it would break other machines/CI. CI regenerates it from the relative path in mops.toml. (The Rust macro PR avoids this because Cargo workspace deps are relative.)
• moc 1.9.0 requirement: mixins raise the minimum moc for consumers — intended, will be documented in the release notes.

Sequencing

A (interface-sync guard, #403) → B (Rust macro, #404) → C (this).

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