feat(analyzer): reduce false positive for emoji-zwj-sequence

[badnoodle]

Fixes #259 , check the valid Emoji ZWJ sequences to reduce false positive

[rng1995]

[Automated SkillSpector Review]

Requesting changes. The emoji carve-out misses common valid ZWJ sequences containing skin-tone modifiers or VS16, and the PR adds no regressions. Handle the full emoji sequence around ZWJ, add representative positive/negative tests, declare regex as a direct dependency, and run formatting.

[rng1995]

[Automated SkillSpector Review]

Re-review of head d2dd3e6. The PR carves ZWJ (U+200D) out of the P2 hidden-instruction rule when it joins a valid emoji sequence (fixes #259). The new commit substantially addresses the sequence-handling feedback and adds good test coverage, but one prior blocker remains open and the new code introduces a crash on attacker-controllable input.

Prior-issue resolution checklist

Prior issue	Status
Handle full emoji sequence around ZWJ (Emoji_Modifier, FE0F), e.g. 👩🏽‍💻, ❤️‍🔥	Resolved — is_emoji_zwj_sequence now walks left past U+1F3FB–U+1F3FF and FE0E/FE0F to the \p{Extended_Pictographic} base; tests cover those exact emoji
Representative positive/negative tests incl. non-emoji ZWJ staying detectable	Resolved — added in tests/unit/test_patterns.py and tests/nodes/analyzers/test_static_patterns.py, incl. ignore\u200dall system\u200d\u200dprompt still yielding P2
Declare regex as a direct dependency	Not resolved — it was added to the dev optional-dependency group in pyproject.toml, but common.py (runtime code imported by every analyzer) does import regex at module level. A plain pip install skillspector now fails at import time. Move it to [project] dependencies
Run formatting	Not resolved — leftover debug print(findings) in a test, test-name typo seuquence, inline-comment style, fullwidth colon in a comment, missing return-type annotations

New blockers

1. Crash on ZWJ near start of file (src/skillspector/nodes/analyzers/common.py). _read returns "" for out-of-range indexes, but _check_modifier/_check_selector call ord(cp) unconditionally, and ord("") raises TypeError. Reproduced: content starting with \u200d (ZWJ at index 0), or a modifier at index 0 followed by ZWJ, crashes is_emoji_zwj_sequence. Neither analyze() nor static_runner.run_static_patterns() catches analyzer exceptions, so a skill file beginning with a ZWJ takes down the entire prompt-injection node — an attacker can prepend one character to suppress all P1–P4 scanning. Guard the empty-string case (e.g. return False from the _check_* helpers when cp is empty) and add a test with ZWJ as the first character of the file.
2. Runtime ImportError in production installs (same as unresolved prior issue above): regex must move from dev extras to [project] dependencies.

Non-blocking nits

• _check_emoji_zwj_sequence: content[match.start() : match.end()] is identical to match.group(0); the condition can simply be match.group(0) != "\u200d". Prefer conventional operand order over Yoda conditions, and add -> bool annotations (repo convention).
• tests/nodes/analyzers/test_static_patterns.py: remove debug print(findings); fix test-name typo test_p2_emoji_zwj_seuquence_still_keep_detectable; zwj_with_modifier actually contains no skin-tone modifier (VS16 only) — rename or use a modifier-bearing sequence.
• Consider precompiling the \p{Extended_Pictographic} pattern at module level instead of per-character regex.match calls.

Once the crash is fixed (with a boundary test) and regex is a runtime dependency, this looks mergeable.

Note for maintainers: PR #264 fixes the same issue #259 with a stdlib-only carve-out in the same hunk — the two PRs conflict and only one should land.