feat: add memo utility

[MathiasWP]

Summary

Adds a new memo utility — the $derived analog of watch.

$derived automatically tracks every signal read inside its expression. That's usually what you want, but sometimes the compute body needs to read state that shouldn't trigger recomputation. Today the only escape hatch is wrapping every unrelated read in untrack, which is verbose and easy to get wrong.

memo flips the model and mirrors watch's API split:

• Arg 1: a getter (or array of getters) that declares what should trigger reactivity. Its return value is only used to register dependencies.
• Arg 2: a zero-argument compute function that produces the value. Reads inside it are not tracked.

let count = $state(2);
const doubled = memo(() => count, () => count * 2);
doubled.current; // 4

let a = $state(1);
let b = $state(2);
const sum = memo([() => a, () => b], () => a + b);

// only `count` triggers recomputation; `multiplier` is read but not tracked
let multiplier = $state(2);
const value = memo(() => count, () => count * multiplier);

Implementation

• packages/runed/src/lib/utilities/memo/memo.svelte.ts — memo function. Touches the source getter(s) to register them as dependencies, then runs the compute body inside untrack so nothing inside it gets tracked. Internally constructs a small class that holds a $derived.by and exposes the value via .current (the standard runed pattern, since runes are compiler macros and can't be returned as bare reactive values from a function).
• packages/runed/src/lib/utilities/memo/memo.test.svelte.ts — 4 tests:
  • recomputes when a tracked source changes
  • does not recompute when an untracked source read inside the compute body changes
  • supports an array of sources
  • only the listed sources trigger recomputation (cross-check that the latest value of an un-tracked read is still picked up on the next recomputation)
• packages/runed/src/lib/utilities/memo/index.ts + packages/runed/src/lib/utilities/index.ts — re-exports.
• sites/docs/src/content/utilities/memo.md — docs page modeled directly on watch.md for consistency (same framing paragraph structure, same example progression: single source → array → untracked-read example).
• .changeset/add-memo-utility.md — minor bump.

Test plan

• pnpm exec vitest --run src/lib/utilities/memo — 4/4 passing
• pnpm check — 0 errors, 0 warnings
• Maintainer review on naming (memo vs alternatives) and on the choice to expose via .current rather than a different accessor

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[changeset-bot]

🦋 Changeset detected

Latest commit: 2ea24d1

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
runed	Minor

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[AndersRobstad]

Very useful! I often end up inlining such with derived and watch