dev: compile integration artifacts into their own directory

[0xisk]

Raised by @andrew-fleming in #717.

Problem

compile:integration writes into the same contracts/artifacts/ tree as src, and the compiler names each artifact directory after the source basename. Two sources sharing a basename silently overwrite each other, last compile wins.

Already happened: ConfidentialFungibleTokenPublicSupply.compact existed under both src/token/extensions and test/integration/_mocks. It surfaced as a wrong-contract arity error, invisible to lint, typecheck, and the truncated-key scan, and was worked around by renaming the mock (f453342d).

Proposed

Give the mocks their own output tree. The compiler already supports it:

-"compile:integration": "compact-compiler --src test/integration/_mocks"
+"compile:integration": "compact-compiler --src test/integration/_mocks --out artifacts/integration"

Then:

• turbo.json — compile:integration outputs become artifacts/integration/**/
• the three test/integration/fixtures/*.ts imports move under artifacts/integration/
• contracts/test-utils/harness/LiveSimulatorBackend.ts:116 hardcodes contracts/artifacts/<name>/ for ZK key loading, so the root has to come from SimulatorConfig instead
• emptyKeyArtifacts's variadic sourceRoots, INTEGRATION_MOCKS (scripts/live/paths.ts), and the #integration conditional in ArtifactCompiler exist only because the trees are shared, and can go

Caveats

• Not net-simpler: it swaps a multi-source-root scan for a multi-artifact-root lookup. The win is that the filesystem enforces the split rather than a naming convention nothing checks.
• Does not fix the turbo output overlap. All nine compile tasks declare outputs: ["artifacts/**/"], so the shared-tree hazard behind dev: scope turbo compile outputs per module (fix 142 GB cache + 0-byte key races) #675 is repo-wide.
• Keep the Composed… mock name. Siblings are ComposedTokens and SharedInitCollision, so the prefix is the convention for these mocks, not a workaround for the clash.
• Touches the live deploy path, so it needs a live run to verify. That is why it stayed out of test(integration): drive the integration specs against the live stack #717.

Cheaper interim

A scripts unit test asserting no basename appears under both src/**/*.compact and test/integration/_mocks/*.compact catches the collision in CI with no live-path change. Worth doing whether or not the move happens.

Refs #716.