feat(data): db.derive inputs overload for external observables

[krisnye]

Adds a second db.derive shape that folds external Observe<T> values (values that don't live in the ECS — e.g. observables exposed by services) into a derive, alongside ECS reads.

// existing — ECS reads only
db.derive(db => db.resources.x + db.resources.y);

// new — inject a fixed record of external observables' CURRENT values
db.derive(
  { scale: someService.scale$ },            // Observe<number>
  (db, inputs) => (db.get(item, 'size') ?? 0) * inputs.scale,
);

• inputs is a fixed record of Observe<T>; the callback's second argument is their current values, keyed the same and unwrapped Observe<U> → U, strongly typed (same mapping as Observe.fromProperties).
• The first argument stays the read projection, so one synchronous body mixes external values with ECS reads.
• Dual trigger: recomputes when any input emits OR a recorded ECS read could have changed. Structurally deduped.
• Cold start: withholds the first value until every input has emitted (fromProperties semantics).
• Static: inputs are subscribed once at the root and fixed for the derive's lifetime — no dynamic service-observe reads inside the body. Anything dynamic (input set depends on data, or an input's args come from an ECS read) composes with Observe.withSwitch / Observe.fromKeys around the derive.

Implemented by composing Observe.fromProperties(inputs) with the existing recording-and-gate — no new machinery. The public surface is two fully-typed overloads; the hidden implementation signature uses any in one param position only because an overloaded impl must stay callable for the typed overload (standard TS idiom; no casts, and the loose derive type already used any).

Tests

• Compile-time (database-read.type-test.ts): inputs unwrapped to current values, db still the read projection; negatives for wrong value type, unknown key, and non-Observe input.
• Runtime (observe-derive.test.ts): inject + dual trigger (input change and ECS read change in one body), two inputs combined with an ECS index read, cold-start withholding, structural dedupe.
• Full data suite green (2823 tests), tsc -b + eslint clean.

Version bumped 0.9.81 → 0.9.82.

🤖 Generated with Claude Code