Add PDF to fax conversion support

- pdf.rs: Ghostscript-based PDF to TIFF-F conversion
- tiff_to_fax_document: Parse TIFF and convert to 1-bit fax format
- CLI: Add 'convert' subcommand for dry testing
- CLI: Add resolution option for send command
- Test files for PDF/TIFF conversion validation

test/testfax.ps

@@ -0,0 +1,41 @@
+%!PS-Adobe-3.0
+%%Pages: 1
+%%PageOrder: Ascend
+%%BoundingBox: 0 0 612 792
+%%EndComments
+
+%%BeginProlog
+/Dict 2 dict def
+Dict begin
+/PageSize [612 792] def
+end
+
+%%EndProlog
+
+%%Page: 1 1
+10 750 moveto
+/Fnt1 12/Times-Roman findfont definefont pop
+/Fnt1 12 selectfont
+(Telfax Test Document - Page 1) show
+
+10 730 moveto
+(This is a test fax transmission from telfax Rust server.) show
+
+10 700 moveto
+(Testing US Robotics 56K FAX USB Modem) show
+
+10 680 moveto
+(Date: 2026-07-01) show
+
+10 600 moveto
+/BoxTimes-Roman 14 selectfont
+(TEST FAX CONTENT) show
+
+10 550 moveto
+(This document tests the Class 1 fax protocol implementation.) show
+
+showpage
+%%PageTrailer
+
+%%Trailer
+%%EOF