Detect extensionless archive formats

[andrew]

Detect ZIP, TAR, gzip, bzip2, and xz content when a filename has no supported archive extension. Keep existing filename mappings primary, preserve OpenBytes zero-copy behavior, and route compressed content through the existing TAR readers.

Add fixtures for both opening APIs, misleading known extensions, generic compressed streams, and extensionless gem content. Add a benchmark for the detected tar.gz path and document the fallback.

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Open now buffers the entire stream before it can reject unsupported inputs, increasing memory/IO cost for large non-archive readers.

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Pull request overview

Adds content-based archive format detection as a fallback when filename-based format detection fails, enabling Open/OpenBytes to handle extensionless inputs while keeping existing extension mappings primary.

Changes:

• Add magic-based content sniffing fallback for ZIP/TAR/gzip/bzip2/xz when no supported filename extension is present.
• Add tests/fixtures covering extensionless inputs, misleading known extensions, generic compressed streams, and extensionless gem content.
• Add a benchmark for the content-detected tar.gz path and document the fallback behavior in the README.

File summaries

File	Description
README.md	Documents filename-first behavior and the content-sniffing fallback.
hash_test.go	Updates OpenBytes zero-copy test to exercise extensionless detection.
hash_bench_test.go	Adds benchmark for the detected tar.gz path via extensionless OpenBytes.
archives.go	Introduces content-based format detection fallback and format constants.
archives_test.go	Adds fixtures and tests for extensionless detection and precedence rules.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 1
• Review effort level: Lite

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