Add .conda package reader

[andrew]

Based on #31.

The v2 conda format is an uncompressed zip containing metadata.json, pkg-<name>.tar.zst and info-<name>.tar.zst. Both inner tarballs already store their entries with the paths that appear in the equivalent v1 .tar.bz2 package (info/ is a prefix inside the tar, not something to add), so openConda opens each .tar.zst member through openTar and concatenates the entries into a single tar view. Hash covers the outer .conda bytes, matching what anaconda.org publishes in repodata.json.

Decompressed size is capped cumulatively across all members so a zip with many pkg-*/info-* entries cannot exceed maxDecompressedSize by staying under it per member.

Verified against six-1.17.0-pyhd8ed1ab_0.conda from conda-forge: 25 merged entries, info/index.json extracts, and Hash("sha256") returns 41db0180… matching the repodata digest.

Closes #24

[Copilot]

Pull request overview

Adds first-class support for v2 conda packages (.conda) by treating them as an outer ZIP that contains pkg-*.tar.zst and info-*.tar.zst, and exposing a merged “tar-like” view via the existing tarReader APIs.

Changes:

• Add a new .conda reader that opens relevant .tar.zst members and merges their tar entries into a single browse/extract view.
• Register .conda in format detection/opening paths and document support in the README.
• Add unit tests covering listing/extracting, hashing the outer archive, and decompression-limit behavior.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
README.md	Documents .conda as a supported archive type and describes how it’s presented.
conda.go	Implements the .conda reader and member parsing/merging logic.
conda_test.go	Adds tests for .conda open/list/extract/hash and size-limit behavior.
archives.go	Registers the new conda format in detection and dispatch.
archives_test.go	Extends TestDetectFormat to include .conda.

Suppressed comments (2)

conda.go:51

• if files == nil treats an archive with matching members but zero tar entries as if the members were missing, and produces a misleading error. Track whether any pkg-/info- .tar.zst members were found instead of relying on whether any entries were appended.

	if files == nil {
		return nil, fmt.Errorf("no pkg-*.tar.zst or info-*.tar.zst member in conda package")
	}

conda.go:57

• The merged view can contain duplicate paths (e.g., both tarballs containing the same directory entry). index keeps only the first occurrence for Extract, but List/ListDir will still surface duplicates from files, which is inconsistent for callers. Consider de-duplicating files by path while building index.

	index := make(map[string]int, len(files))
	for i, f := range files {
		if _, seen := index[f.info.Path]; !seen {
			index[f.info.Path] = i
		}

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