ptx: fix panic when reference length exceeds line width

[CrazyRoka]

Description:

Fixes a panic in ptx where subtraction overflow occurred if the generated reference string (filename + line number) was longer than the specified line width (e.g., ptx -w 1 -A long_file).

thread 'main' (213854) panicked at src/uu/ptx/src/ptx.rs:750:9:
attempt to subtract with overflow

Fix:

Changed direct subtraction config.line_width -= max_ref_len to saturating_sub. If the reference consumes the entire line, the available width for context simply becomes 0.

Tests:

Fixes failure in GNU test tests/ptx/ptx-overrun.sh.
Added new regression test test_narrow_width_with_long_reference_no_panic.