Support dictionary-compressed virtual chunk locations

[Shane98c]

Closes #3. Closes #21.

Decodes icechunk's dictionary-compressed virtual chunk locations (Manifest.location_dictionary + ChunkRef.compressed_location, from earth-mover/icechunk#1776).

Why a vendored, patched fzstd?

compressed_location needs zstd dictionary decompression, which released fzstd@0.1.1 lacks. Tested against all 244 real metadata fixtures:

Decoder	Result
fzstd@0.1.1	byte-exact on all 244, but no dictionary API
zstdify (1.4.0 and main)	silently corrupts 77/244 (~32%) — wrong bytes, no error (vs zstd CLI)
fzstd PR#18	correct on all 244 + decodes real dictionary frames

zstdify is disqualified (silent corruption), and dictionary support exists only in the unmerged, unreleased fzstd#18 (can't be cleanly npm installed; upstream dormant). So we vendor its source under src/vendor/fzstd/ (MIT, attributed — see its README.md for provenance, re-vendor steps, and drop criteria).

Testing

New manifest-dictionary.test.ts covers end-to-end decode, missing-dictionary error, the size bound, the aliasing-fix regression, and malformed-frame rejection. Fixtures are zstd-CLI-generated (no upstream fixture uses this feature yet).

[gemini-code-assist]

Code Review

This pull request vendors the fzstd library to support zstd dictionary decompression for virtual chunk locations in manifests, resolving dependency limitations and data corruption issues. It updates the manifest parser to decode compressed locations using the manifest's zstd dictionary, implements safety checks to bound decompression memory allocation, and aligns payload extraction priority with the Rust implementation. Comprehensive test coverage is also added. Feedback on the changes points out a potential integer overflow in zstdFrameAllocSize due to 32-bit bitwise shift operations in JavaScript, suggesting standard arithmetic exponentiation and division as a safer alternative.

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[gemini-code-assist]

In JavaScript, bitwise operators (like << and >>) operate on 32-bit signed integers. If the window descriptor wd has a large exponent (e.g., wd >> 3 >= 21), 10 + (wd >> 3) will be >= 31, causing 1 << (10 + (wd >> 3)) to overflow or wrap around to a negative or incorrect value. Similarly, base >> 3 will overflow if base is >= 2^31.

To prevent this and ensure correct handling of large window sizes (especially when validating untrusted input), use standard arithmetic exponentiation (2 ** ...) and division (/ 8) instead of bitwise shifts.

Suggested change

	const base = 1 << (10 + (wd >> 3));
	windowSize = base + (base >> 3) * (wd & 7);
	const base = 2 ** (10 + (wd >> 3));
	windowSize = base + (base / 8) * (wd & 7);