file: add the compression-aware identification CLI

[brandonpayton]

Purpose

Migrate the registry file package into Homebrew as the command-line consumer of the complete tap libmagic database. The CLI follows Homebrew's standard file-formula name because File is a reserved Ruby class; the tap alias file preserves the expected user-facing package name.

This removes the registry package's 14,000-line magic.lite workaround. Normal file invocations now load the full compiled database from the stable libmagic opt path.

Implementation

• builds only upstream file(1), replacing the sibling libmagic.la dependency with the installed static tap contract
• requires the matching libmagic 5.45 version
• carries explicit static compression dependencies and validates pkg-config propagation
• instruments the final Wasm executable for its retained fork/posix_spawn fallback path
• installs only bin/file and file(1); libmagic remains the sole owner of the library and database
• adds the standard tap alias file -> file-formula

Evidence

• source reinstall passed in an isolated ABI-18 Homebrew prefix
• literal brew test --keep-tmp --verbose passed
• Node tests cover the default database, WebAssembly recognition, PDF MIME output, stdin, symlink/no-dereference and -L behavior, plus zlib, gzip, bzip2, and xz -z decoding
• Chromium BrowserKernel produced the same WebAssembly/PDF/compression classifications with guest PATH=/no-such-path
• the installed binary contains only the libmagic opt database path
• direct Wasm inspection shows kernel_fork, all five wpk_fork exports, no Asyncify exports, and __abi_version returning 18
• clean-prefix brew audit --strict and brew style passed
• git diff --check passed

Dependencies

This PR is intentionally stacked on #29, which is stacked on compression-library PR #25.

Platform gates before bottle publication/merge:

• sdk: reserve an 8 MiB main-thread shadow stack for linked programs kandelo#842
• Build fork instrumentation from the repository workspace kandelo#873
• sdk: centralize cross-compile function availability kandelo#874
• ci: isolate and verify Homebrew bottle publication kandelo#881
• Reject stale Wasm artifacts with aliased ABI exports kandelo#882
• homebrew: publish tap-native formulas without registry duplicates kandelo#885
• Test fork instrumentation output reproducibility across processes kandelo#888
• homebrew: trust same-tap dependencies during bottle builds kandelo#890
• ci: add thin tap workflows for bottle publishing and maintenance #22

The pthread fork-continuation fix and ABI 18 transition from #867 are merged. Final bottle order is bzip2/xz (#25), libmagic (#29), then this File consumer; the poured chain must pass stock guest Node and Chromium validation.

The branch was transplanted semantic-preservingly onto current libmagic head 991712b at File head 5735ce1. The stable patch ID and Formula/Alias/README blobs are byte-identical to the functionally validated head. Post-restack validation through scripts/dev-shell.sh passed Homebrew Ruby syntax, the current support suite (21 runs/186 assertions), brew style, and git diff --check.

Do not merge until the dependency stack and platform gates are ready. No pre-final bottle is accepted for publication.

[brandonpayton]

Independent devil review found no blockers.

The review specifically confirmed:

• FileFormula plus the tracked Aliases/file symlink is the correct reserved-name handling
• static dependencies and pkg-config propagation are explicit
• the formula builds only the CLI against external libmagic and installs only file plus file(1)
• the stable libmagic opt database path, skip_clean, and final fork instrumentation are present
• tests cover default DB, MIME, stdin, symlink semantics, zlib, gzip, bzip2, and xz
• BSD-2-Clause-Darwin matches the CLI source license

Residual: the committed Node compression test inherits PATH. Built-in-only proof therefore rests on the recorded Chromium run with guest PATH=/no-such-path plus the underlying libmagic runtime test; the reviewer considered that evidence adequate.

[brandonpayton]

Replaced by #69 after libmagic merged. GitHub auto-closed this stacked PR when its base branch was deleted, and then marked it non-reopenable once the File head was transplanted onto current main. #69 contains the same File formula body plus the canonical installed-tap loader, directly based on main.