add range coalescing option

[Shane98c]

Adds opt-in range coalescing for icechunk chunk payload reads.

• New src/reader/range-coalescer.ts adapters expose icechunk's two read paths as zarrita AsyncReadable:
  • makeStorageStore — native chunks via Storage
  • makeUrlStore — virtual chunks via direct HTTP, with optional conditional headers for validateChecksums
• ReadSession keeps a bounded LRU of coalescer-wrapped stores, partitioned by URL, fetch client, and checksum identity so conditional headers and per-client state never bleed across a coalesced batch.
• Zarrita remains a true optional peer dep — the adapters mirror its AsyncReadable shape locally and only run user-supplied withRangeCoalescing.
• Handles servers that ignore Range (HTTP 200 fallback) by slicing the requested window out of the full body, plus suffix-length range support.

[gemini-code-assist]

Code Review

This pull request introduces range coalescing for chunk payload reads by integrating with zarrita's withRangeCoalescing utility. This allows concurrent reads from the same backing object to be merged into fewer, larger requests, improving efficiency for both native and virtual chunks. The implementation includes a new range-coalescer.ts module, an LRU cache for coalesced stores in ReadSession, and updates to the IcechunkStore API to expose the rangeCoalescing option. Feedback is provided regarding the restoration of range details in error messages for native chunk fetch failures to maintain observability.

[gemini-code-assist]

The error message for native chunk fetch failures has been simplified, removing the specific range information. While cleaner, this might make debugging harder if a storage backend returns an unexpected length. Consider keeping the range details in the error message for better observability.

Suggested change

	throw new Error(
	`Storage returned ${data.length} bytes for ${path} range ${range.start}-${range.end - 1}; expected ${payload.length} bytes`,
	`Storage returned ${data.length} bytes for ${path}; expected ${payload.length} bytes`,
	);
	throw new Error(
	`Storage returned ${data.length} bytes for ${path} range ${payload.offset}-${payload.offset + payload.length - 1}; expected ${payload.length} bytes`,
	);