fix(builtins): change diff default output to normal format

[chaliy]

Summary

• Changed diff builtin default output from unified to normal (ed-style) format, matching /usr/bin/diff
• Added format_normal() implementation for ed-style output (change/add/delete commands)
• -u flag still produces unified format

Why

Scripts that parse diff output expecting the default format got wrong results because bashkit defaulted to unified format while real bash/diff defaults to normal format.

Tests

• Updated all existing unit tests to expect normal format by default
• Added test_diff_normal_format_default — verifies ed-style output and absence of unified markers
• Added test_diff_unified_with_flag — verifies -u still produces unified format
• Added spec tests: diff_default_normal_format and diff_unified_with_flag
• 100% bash comparison match (1692/1692)

Closes #966