feat(sim): behaviours, session model, engine

[Kinflou]

Milestone 1c of playground simulation. The sim is now driveable end to end from a script — no UI yet (that's 1d).

behavior.ts — six canned behaviours

Each is a spec (label, appliesTo, defaultConfig seeded from the function's return / error types, make):

kind	does
Reply with value	returns the configured value (default for a normal fn)
Echo params	returns the decoded params
Increment field	returns a base with one numeric field bumped per call
Delay then reply	waits N ms, then replies
Raise error	reply.err(ordinal, data) for one of the fn's throws
Drop	never settles — the client's call stays pending (default for a one-way fn)

model.ts — the session

Session { shape, instances, connection }. addInstance (auto-named <proto>-N; server instances get a seeded behaviour map), removeInstance (drops the connection if it referenced the instance), setConnection (same protocol + opposite roles, replaces any existing), setBehavior, and rebuild — re-point at a freshly compiled shape, keeping surviving instances and the configs of functions that remain.

engine.ts — bring a connection live

connect(session, connection) → a GenericDispatch server bound to the instance's behaviours + a GenericClient, over a tapped duplex(), handshake run. setBehavior(fnName, setting) swaps a live entry (mutates the BehaviorMap GenericDispatch reads per dispatch) — takes effect on the next call.

Tests

✔ model — instance naming, seeded behaviours, removal drops the connection
✔ Reply with value returns the configured value
✔ Echo returns the params; a live behaviour swap takes effect on the next call
✔ Increment field bumps once per call
✔ Raise error comes back as SimRemoteError mapped to the ordinal's name
✔ Drop leaves the call pending
✔ rebuild keeps a surviving instance's behaviour config

12/12 sim tests pass (7 new + 5 from 1b). npm run build green; no app-bundle change (nothing imports sim/behavior|model|engine from an entry yet).