Add the Runtime value, lay out the core's layers, and add the harness

[andreisavu]

PR 1 of the stack that SPLIT_PLAN.md on #48 cuts from that branch. No behaviour change.

The one idea

A host owns a Runtime, and the core is laid out in layers that Biome holds.

• host/runtime.ts. The clock, the session opener, the model call, the catalog, the rooms that run and the workspace names that are taken live on one value. startSession, readSession and defineWorkspace take a Runtime and default to defaultRuntime. Every Date call in src routes through the runtime's clock, and the view a seat reads carries the time it was built at. session.ts keeps its shape: only its globals move into the runtime it is handed.
• The layers. types, define, render are the vocabulary; host/ holds what a host owns; tools/ holds the workspace and its backends; session.ts composes them. biome.jsonc carries one noRestrictedImports override per layer, as the final tree has it, plus the two cross-package rules. The overrides for wire, log/, room/ and seat/ name directories no file is in yet; they are harmless until PR 2 and PR 3 fill them.
• types.ts gains the host contracts (Clock, SessionOpener, ModelResolver) and BUILTIN_TOOL_NAMES, so the vocabulary imports nothing that does anything. Backlog items 1 and 5 close.

The tests that pin it

• test/runtime.test.ts: two runtimes in one process run the same name and neither reads the other; a room on JSONL reads back through a second runtime over the same directory.
• test/matrix.test.ts: four scenarios (test/support/scenarios.ts) on both storages (memory, jsonl), on a clock the test moves by hand, held to the shared invariants (test/support/invariants.ts). The live tier re-exports the same invariants.
• The existing session, roster, presence, assistant and workspace suites pass unchanged, which is the point.

Extract

git cherry-pick -x 90d5871 from #48, then runtime.ts moved to host/, the three workspace files moved to tools/, the contracts moved to types.ts, and the Biome overrides added. Trimmed per the plan: Transport, the transport option, wake, retry, evict, checkpoint on the runtime; SQLite, tappedOpener and faultyOpener in the test storage; inherited and the lease check in the invariants. One deviation: wrangler.jsonc is left out of the test task's inputs in turbo.jsonc until PR 14 adds the package that has one.

What a later PR adds on top

• PR 2: log/log.ts in place of record.ts, the serial commit queue.
• PR 3: wire.ts, seat/, Transport and RunningRoom as the wire sees it.
• PR 5: wake, retry and evict on the runtime; resumeSession.
• PR 6: the faulty and tapped openers, the chaos tier.
• PR 13: the SQLite storage and the third column of the matrix.

Checks

pnpm format && pnpm check is green. The build prints three TS4023 lines from test/live/support.ts that main prints too; they are outside this PR. Every doc link resolves, and the vocabulary override was checked to refuse an import of ./host/runtime.ts from types.ts.

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