feat(sdk/script): classify signal.error/interrupt against errdefs and engine.Cause

[lIang70]

Summary

Script signal.error(msg) returned a flat string, leaving hosts (scriptnode and any future script-based engine) unable to tell a validation failure apart from a budget refusal or an internal crash without parsing the message. signal.interrupt(msg) had the same gap, always collapsing to engine.CauseCustom.

This PR makes the signal protocol natively typed and aligns it with errdefs / engine.Cause:

• script.Signal gains Kind (errdefs category for errors, engine.Cause for interrupts) and Detail (freeform structured metadata).
• New script.SignalToError is the single point of truth that maps a signal into either errdefs.<Kind>(message) or engine.Interrupted({Cause, Detail}). Unknown kinds degrade safely (Internal / CauseCustom) and preserve the raw value in the error chain so observability surfaces the typo.
• jsrt and luart accept either a bare string (back-compat, kind stays empty) or an object/table { kind, message, detail } for both signal.error and signal.interrupt. The two implementations stay textually parallel so behaviour cannot drift.
• sdk/graph/node/scriptnode/jsnode.go drops its inline switch sig.Type and delegates to script.SignalToError, %w-wrapping with the node id so errdefs.Is… and errors.As(InterruptedError) still work through the chain.
• iteration.js forwards Kind / Detail when re-raising a child signal so the child's classification reaches the host instead of being flattened.

Script-facing API (no breaking changes)

```javascript
// Back-compat: bare string → Internal / CauseCustom
signal.error("boom");
signal.interrupt("pause");

// New: typed
signal.error({ kind: "validation", message: "model is required", detail: { field: "model" } });
signal.interrupt({ kind: "user_input", message: "barge" });
```

Exposed ErrorKind allowlist (deliberate subset of errdefs)

`validation`, `not_found`, `budget_exceeded`, `policy_denied`, `not_available`, `internal`.

Auth / rate-limit / timeout / conflict are intentionally NOT script-raisable — they are wire-level classifications that belong in the Go layer.

Test plan

• `go vet ./sdk/...`
• `go test ./sdk/...` — all packages pass (script, script/jsrt, script/luart, script/bindings, graph/node/scriptnode, etc.)
• `go test ./sdkx/... ./vessel/... ./voice/... ./cmd/vesseld/...` — no consumers broke
• New `sdk/script/signal_test.go` covers every errdefs kind, every `engine.Cause`, empty-kind defaults, unknown-kind degradation, back-compat bare strings
• `jsrt` / `luart` tests cover the object/table form for both signal.error and signal.interrupt and assert Kind / Detail flow through
• scriptnode integration tests assert a kinded `signal.error` reaches the host as `errdefs.IsValidation` and a kinded `signal.interrupt` preserves `engine.Cause=user_input`

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