feat: reactive getConfig()

[jimmy-phantom]

Summary

Replaces PR #20's per-fetch subscribe rebuild with a signalium-native reactive model. getConfig() re-runs only when the response values it actually reads have changed, not on every fetch. For poll-like subscribers this collapses steady-state rebuilds to zero; for stateful subscribers (websockets, EventSource) it eliminates the reconnect-per-fetch cost.

User-facing API

import { reactiveSignal } from 'signalium';

class GetPrice extends RESTQuery {
  path = '/price';
  result = { value: t.number };

  getConfig() {
    const ok = reactiveSignal(() => {
      this.responseNotifier.consume();
      return this.response?.ok;
    }).value;

    return {
      subscribe: poll({ interval: ok ? 1000 : 5000 }),
    };
  }
}

Two patterns at the read site:

• Plain (non-reactive): this.response?.X. Read once when getConfig() first evaluates, never re-read. Use when no reactivity is needed.
• Reactive: reactiveSignal(() => { this.responseNotifier.consume(); return this.response?.X; }).value. Re-evaluates after every fetch (when responseNotifier fires). reactiveSignal filters propagation via Object.is on the thunk's return value, so extract a primitive (e.g. response.ok, response.status) rather than returning a fresh object or the Response itself, otherwise getConfig re-runs on every fetch even when nothing meaningful changed.

The framework adds nothing beyond responseNotifier; consume() + plain this.response reads is the user pattern. Headers and other derived values use the same shape (this.response?.headers.get('etag') inside the thunk).

Framework changes

• ctx.response stays a plain Response | undefined field. New ctx.responseNotifier (signalium Notifier) initialized in createExecutionContext.
• RESTQueryAdapter.executeRequest writes both ctx.response = fetchResponse and ctx.responseNotifier.notify() after each fetch.
• this.config and this.retryConfig on QueryInstance are getters backed by _resolvedOptions = reactiveSignal(() => resolveOptions(ctx)). The signal's dep tracking filters propagation; cached value returned when no tracked signal changed.
• reconcileSubscription ref-checks against lastSubscribeFn before tearing down. Identical-subscribe reads no-op.
• lastSubscribeFn cleared on deactivate and on paramsDidChange so those paths always rebuild.

Test plan

• Two new failing-repro tests in poll.test.ts (honors a state-dependent interval after first fetch resolves, stops polling when getConfig() switches subscribe to undefined after an error) pass with surgical behavior verified.
• Full test suite: 1232 passed, 2 skipped.

Footgun worth flagging

A user can write reactiveSignal(() => this.response?.ok).value and forget the consume() call. The derived signal would compute once with no deps and stay cached forever, silently producing stale config. Mitigation candidates: documentation convention ("always call this.responseNotifier.consume() at the top of a reactiveSignal thunk that reads this.response") and an optional lint rule.

🤖 Generated with Claude Code