Scientific numeric tiles

Scientific readers continue to emit RasterBlock: bounded canonical big-endian bytes that are portable across browsers, Node.js, storage, and future RPC boundaries. Repeated computation should use NumericTile one layer above that boundary. A numeric tile contains native-endian typed data, component and layout metadata, element strides, and an idempotent release() callback.

rasterBlockToNumericTile() preserves the source numeric type by default. Canonical float16 expands to Float32Array; canonical uint64 remains exact in BigUint64Array. A caller may request a checked target type, but conversion rejects overflow or precision loss rather than silently rounding. Preserved uint8 and int8 blocks are zero-copy when no destination or allocator is supplied. Wider preserved values can only be zero-copy on a big-endian host with aligned byte and stride boundaries; common little-endian hosts perform one canonical-to-native conversion.

Reusable storage is explicit. Pass a destination typed array or a caller-owned NumericTileAllocator; there is no package-global pool. For copied tiles, source-block ownership is released as soon as conversion completes and tile release returns only allocator-owned storage. For a zero-copy tile, tile release propagates to the source block. Both paths are idempotent and release on conversion or cancellation errors.

Dataset bridge and direct sources

scientificDatasetToNumericTileSource(dataset) permanently adapts any labeled-axis ScientificDataset. It preserves lazy region reads and converts each emitted block once during a consumer pass. resolveNumericTileSource(dataset, options) uses an explicitly attached numericTileSource only when its declared source type, native type, component count, layout, and target-type support match exactly; otherwise it selects the portable adapter.

A future WASM or specialized reader opts in by implementing DirectNumericTileDataset and attaching a NumericTileSource with exact directSemantics. Application code keeps calling the resolver and does not import or detect the provider. The direct source remains responsible for honoring the same region, cancellation, metadata, and release contract. The source object is an ordinary local capability: importing PureJsImage does not register it globally, and it is not a sandbox.

A future tile cache can wrap a NumericTileSource and supply its own allocator/lifetime policy. This PR intentionally adds no shared cache or long-lived retention.