feat(8.0): EntityIdMapper U32 ceiling + EntityIdSpaceExceeded error

The JS fallback EntityIdMapper caps at u32::MAX to match the metadata
index's Roaring32 bitmap width. Before this guard, a brain past 4.29 B
entities would silently widen `nextId` into JS's safe-integer range,
truncating ints inside the bitmaps and zeroing query results — the
same silent under-count cortex 3.0's Piece 10 just closed on the
native path. Brainy 8.0 surfaces the overflow loudly instead.

When `nextId` would exceed `U32_ENTITY_ID_MAX`, `getOrAssign` throws
`EntityIdSpaceExceeded` with a message pointing at the cortex 3.0
binary mapper's `idSpace: 'u64'` mode as the migration path (mmap-
backed extendible-hash KV, persists the full u64 range losslessly).

The existing-UUID lookup path bypasses the guard — only fresh
allocations can overflow.

Surfaces via `@soulcraft/brainy/internals`:
- `EntityIdMapper` (already exported, behavior gated)
- `EntityIdSpaceExceeded` (new)
- `U32_ENTITY_ID_MAX` (new constant, = 0xFFFF_FFFF)

This is the lockstep half of cortex 3.0 / Piece 10 / Step 15. Cortex's
`NativeBinaryEntityIdMapperWrapper` ships the U64 binary mapper that
takes over above this ceiling; brainy ships the bright-line failure
that points consumers at it.

New test: tests/unit/utils/entity-id-mapper-u32-ceiling.test.ts (6
assertions covering the constant, the error shape, normal
allocations, the boundary case at exactly u32::MAX, the overflow
throw, and the existing-uuid lookup bypass).

src/internals.ts

@@ -7,7 +7,11 @@ export type { UnifiedCacheConfig, CacheItem } from './utils/unifiedCache.js'
 export { prodLog, createModuleLogger } from './utils/logger.js'
 export { FieldTypeInference, FieldType } from './utils/fieldTypeInference.js'
 export type { FieldTypeInfo } from './utils/fieldTypeInference.js'
-export { EntityIdMapper } from './utils/entityIdMapper.js'
+export {
+  EntityIdMapper,
+  EntityIdSpaceExceeded,
+  U32_ENTITY_ID_MAX,
+} from './utils/entityIdMapper.js'
 export type { EntityIdMapperOptions, EntityIdMapperData } from './utils/entityIdMapper.js'
 export { getRecommendedCacheConfig, formatBytes, checkMemoryPressure } from './utils/memoryDetection.js'
 export type { MemoryInfo, CacheAllocationStrategy } from './utils/memoryDetection.js'