feat: add pipe mode and composable --strip/--halt action flags

[mit-d]

Summary

• Pipe mode (check-unicode -): reads stdin line-by-line and writes to stdout, enabling use as a streaming Unix filter for log monitoring, CI pipelines, and editor buffer filtering
• --strip [dangerous|all]: removes non-ASCII characters from output; dangerous strips only invisible/bidi chars, all (default) strips any remaining non-ASCII after allow-list processing
• --halt [dangerous|all]: stops immediately on first matching character; dangerous (default) halts on invisible/bidi chars, all halts on any non-ASCII
• All three action flags (--fix, --strip, --halt) are fully composable and work identically across file and pipe modes

Examples

check-unicode - < file.txt                    # check stdin, passthrough to stdout
check-unicode --fix --strip dangerous -       # fix fixable, strip bidi attacks
check-unicode --halt - < input.txt            # halt on first dangerous char
tail -f app.log | check-unicode --fix -       # stream-filter logs

Design

Processing order per character: allow-list → fix → halt → strip

• Pipe mode streams line-by-line with per-line findings on stderr and flushed stdout
• --halt stops mid-file/mid-stream — the triggering file is never modified
• Allow-lists (including per-file overrides) are respected by all flags
• Spec: docs/specs/2026-03-29-pipe-mode-and-action-flags-design.md

Test plan

• 333 tests passing (68 new), covering all flag combinations in both file and pipe modes
• Parametrized integration tests for flag interactions with config files, overrides, severity, --quiet, --no-color
• ruff, ty, and all pre-commit hooks clean
• Manual smoke tests with piped input (--fix, --strip, --halt combinations)