feat: transparent gzip input (.gz files and stdin) - #223

Add transparent gzip decompression for .gz files and gzip-magic stdin
using std.compress.flate.Decompress (gzip container). No new dependencies.

- Files: .gz extension detection, auto-format from inner extension
  (data.csv.gz → csv, table )
- Stdin: peek(2) magic sniff (0x1f 0x8b), non-consuming
- Streaming decompress into existing buffered reader
- Table name derived from inner name (strip .gz before stem)
- Clear error on corrupt gzip stream (fatal + csv_error)
- Non-gzip path unchanged (no sniffing overhead beyond 2 bytes)

Closes #220

- args.zig: path literally ".gz" produced an empty table name; fall back to
  the full path stem when stripping the extension collapses it.
- main.zig: wrap the flate reader so error.ReadFailed is translated into a
  message naming the specific gzip failure (EndOfStream, WrongGzipChecksum,
  etc.) instead of a generic "failed to parse CSV". The wrapper forwards to
  flate's vtable methods and mirrors its buffer cursor so the shared-buffer
  reader keeps working; the previous blind forward hung on empty fill slices.
- build.zig: corrupt-gzip integration test now uses a real .gz name (the old
  fixture ended .gz_truncated and never hit the gzip path) and asserts the
  new error message.