feat(storage): add persistent state and storage backends

AGENTS.md

@@ -113,6 +113,7 @@ this step by step for bug fixes.
   /pooling                 # Object pooling
   /rendering               # Rendering system
   /states                  # Game states (createGameState), inState/onEnter/onExit run conditions, state-scoped entities
+  /storage                 # StorageBackend (localStorage, memory) and createPersistentState: typed records kept outside the game
   /text                    # MSDF font atlas loading and text rendering
   /timer                   # Timer utilities
   /ui                      # Retained-mode UI (anchored rect tree layout, canvases, panels, labels, buttons, focus navigation, toggles, sliders, progress bars, dropdowns, layout groups, content size/aspect ratio fitters)

CHANGELOG.md

@@ -14,6 +14,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 #### Added
 
+- **storage:** New `@forge-game-engine/forge/storage` module for data that has to outlive the page, such as settings, achievements and unlocked levels. `await createPersistentState(name, defaults, { validators, storage })` loads a named record of numbers, strings and booleans once, takes the default for every field that isn't stored, and stores only the fields you `set`, so changing a default in an update reaches players who never changed that field. `set` and `reset` raise `onChange` and return a promise that settles once the change is stored; writes go out one at a time and in order. Nothing falls back silently: a stored value of the wrong type rejects with `PersistentStateValueError` naming the field, an entry that isn't a JSON object rejects with `PersistentStateFormatError`, and storage failures reject with `StorageError` (`StorageUnavailableError`, `StorageBlockedError`, `StorageFullError`). Records are kept through a `StorageBackend`: `createLocalStorageBackend()` (the default) or `createMemoryStorageBackend()`, or your own. See the new Storage guide and Persistent State demo
 - **physics:** Continuous collision detection. Register the new `createContinuousCollisionEcsSystem()` directly after `createEulerIntegrationEcsSystem` and a fast dynamic `CircleCollider` body no longer sinks deep into, or passes through, a static collider (polygon, circle or terrain) between two ticks. A wheel coming down from a jump, for example, now stops at the terrain's surface instead of sinking 25-30 units into it in a single tick. Write teleports to `position.local` before `createTransformEcsSystem` runs, or the sweep treats the jump as motion. The sweeps it uses are public too: `sweepCircleCircle`, `sweepCirclePolygon` and `sweepCircleTerrain` return the first `SweepHit` (`point`, `normal`, `t`) of a circle moving from one position to another
 - **physics:** Collision filtering. `addColliderComponent` takes a `category` (the bits a collider belongs to, default `1`) and a `mask` (the categories it collides with, default `allCollisionCategories`). Two colliders are tested only when each one's category is in the other's mask, so pairs your game would ignore, such as bullets against bullets, never reach the narrow phase
 - **physics:** `raycast` takes a `mask` option, so a ray only hits colliders whose `category` is in it

design/audio-mixer.md

@@ -6,7 +6,7 @@
 | **Kind**                              | Missing feature                                                                                                                                                                                                                                           |
 | **Found in**                          | Galactic Journey demo: `src/audio/audio-mixer.ts`, `src/speed/create-speed-sounds.ts`, `src/explosions/create-explosions.ts`, `src/gun/gun.system.ts`, `src/enemy/enemy.system.ts`, `src/music/create-music.ts`, `src/main-menu/create-settings-panel.ts` |
 | **Engine version at time of writing** | `0.25.8`                                                                                                                                                                                                                                                  |
-| **Related**                           | [`demo-findings.md`](./demo-findings.md), [`persistent-preferences.md`](./persistent-preferences.md)                                                                                                                                                      |
+| **Related**                           | [`demo-findings.md`](./demo-findings.md)                                                                                                                                                                                                                  |
 
 ## 0. Targeted modules
 
@@ -95,8 +95,8 @@ Howler is removed as a dependency.
 ### Out of scope
 
 - **Saving the player's volume settings.** The mixer exposes the values; a
-  game stores them with its other settings
-  ([`persistent-preferences.md`](./persistent-preferences.md)).
+  game stores them with its other settings in a persistent state
+  (`createPersistentState` in `@forge-game-engine/forge/storage`).
 - **Positional (panned or attenuated) audio.** A follow-up once sounds are
   entity-bound (Phase 2), not needed by the demo.
 - **Effects on buses** (filters, reverb, compression, ducking). A bus is