feat: add decode_evasion, detect_scripts, is_mixed_script

[Fieldnote-Echo]

Summary

• decode_evasion(text, *, max_layers=3) — Standalone pre-processor that iteratively peels URL encoding, HTML entities, and hex escape layers from untrusted text. Stops when a full pass produces no changes or max_layers is reached. Never logs decoded content.
• detect_scripts(text) — Returns the set of script buckets (latin, cyrillic, greek, arabic, hebrew, armenian, cherokee, cjk) present in text. Unknown scripts silently ignored.
• is_mixed_script(text) — Returns True when 2+ scripts are detected. Pure analysis — no transformation, no blocking.

All three are opt-in primitives — the core clean() pipeline is unchanged. Zero new dependencies (stdlib only: html, urllib.parse, unicodedata).

Design decisions

• No base64 in v1 — too high false-positive risk; URL + HTML + hex covers 90% of bypass vectors
• Single cjk bucket — Chinese, Japanese (Hiragana/Katakana), and Korean all map to one bucket
• Layer counting per pass — URL→HTML→hex in sequence = one layer, not one per decoder
• max_layers <= 0 fast path — early return, no surprises with negative values
• Script lookup via name.split(" ", 1)[0] — dict lookup on first token of unicodedata.name(), not prefix loop

Test coverage

• 62 new tests (34 script detection, 28 decode evasion) — 303 total, all passing
• Phishing scenarios: pаypal.com, gооgle.com, аpple.com (Cyrillic lookalikes)
• Multi-layer nesting: double/triple/quadruple URL encoding, URL+HTML nesting
• Edge cases: invalid encodings, partial hex, negative max_layers, empty strings
• Integration: decode_evasion → clean → detect_scripts full pipeline composition
• Log safety: decoded content never appears in warning messages

Test plan

• uv run pytest tests/ -v --benchmark-disable — 303 passed
• uv run mypy --strict src/navi_sanitize/ — no issues
• uv run ruff check src/ tests/ — all checks passed
• uv run ruff format --check src/ tests/ — all formatted
• Pre-commit hooks pass (trailing whitespace, ruff, mypy)

🤖 Generated with Claude Code

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve malformed percent-encodings in URL decode

decode_evasion() documents that invalid encodings should pass through unchanged, but _decode_url() calls urllib.parse.unquote() with default UTF-8 errors='replace', so malformed byte sequences are mutated (for example, %FF becomes �). This means attacker-controlled malformed inputs are silently rewritten instead of preserved, which can corrupt values and change downstream sanitization/audit behavior in flows that rely on exact pre-clean text.

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