Deduplicate concurrent manifest fetches

[Shane98c]

This fixes bursts of duplicate manifest requests I was seeing in zarr-layer in some virtual datasets.

• Add a generic single-flight cache helper for concurrent cache-aside loads.
• Use it for manifest reads so parallel chunk lookups share one in-flight manifest fetch per manifest ID.

[gemini-code-assist]

Code Review

This pull request introduces a singleFlight utility to coalesce concurrent cache misses for the same key into a single fetch operation, specifically for manifest loading in ReadSession. This optimization reduces redundant network requests during fan-out reads. Feedback suggests improving compatibility for environments without global DOMException and refining promise initialization to avoid type-casting hacks.