Add directory support to the CLI via SQAR archive streams

[paigejulianne]

Extend squish c/d/s to accept a directory as well as a single file. A directory is serialized into a single SQAR archive stream (files, subdirectories, empty directories, and unix permission bits) and compressed as one unit; decompression detects the archive and recreates the tree under the output path.

The compression engine and libsquish ABI are untouched — the archive format is a CLI-level wrapper compressed as an ordinary SQ02/SQ01 stream. Single-file compression is unchanged and byte-for-byte compatible with earlier releases; auto-detection on decompress keys off the SQAR magic. Extraction rejects absolute paths and .. traversal, and entries are stored sorted so archives depend only on the tree, not on filesystem iteration order.

Documented as docs/FORMAT.md §12; CI gains directory round-trip steps (including empty-dir and SFX-directory checks) on the Linux and Windows-MSVC jobs.

Summary

Format impact

• This change does not affect the compressed format
• This change affects the compressed format, and the magic number was
  bumped (see docs/FORMAT.md)

Testing

• make test passes
• Benchmarks run (bench/run_squish.py) with before/after numbers, if
  this touches compression ratio or speed:

<paste before/after numbers here, if applicable>

Checklist

• Code follows the existing style in the files touched
• Builds clean with -Wall -Wextra (default Makefile flags)
• Public API changes are documented in squish.h and
  docs/API.md
• New source files carry the GPLv3 header used elsewhere in the repo