feat(translation): accept/reject gate on Interpreter's detection confidence (#1376)

Interpreter now returns a real per-detection confidence (source stays "auto"),
so curator can reject the mis-detections it was blindly storing — e.g. a short
English title mis-labelled as German and rewritten into the archive.

The gate consumes ONLY Interpreter's own reported detection — curator does no
language detection of its own (Scribe rule 133): a field is stored when the
engine actually translated it AND either the detected language is CJK
(script-detected, reliably high — ja/ko/zh trusted outright, incl. pure-kanji
Japanese that lands as zh ~0.75) or the reported confidence clears a
latin-script floor (_MIN_LATIN_CONFIDENCE = 0.90). A latin detection below the
floor keeps the original and marks the post handled; a missing confidence fails
open. The client already sent source="auto" and parsed confidence, so this is
purely the gate + tests.

Tests: pinned interpreter-client test now asserts source stays "auto"; new
pure-unit gate tests (CJK trusted / latin floor / case-insensitive / fail-open)
in the fast lane; end-to-end reject-low-latin, accept-high-latin,
accept-low-cjk sweeps.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CgZP9v2otxVJymiYsnVuMy

tests/test_interpreter_client.py

@@ -38,6 +38,9 @@ def test_translate_maps_batch_in_order(monkeypatch):
     assert out["engine_version"] == "ollama:x:12b"
     assert captured["json"]["q"] == ["ねこが可愛い", "金髪ギャル！"]
     assert captured["json"]["target"] == "en"
+    # Source MUST stay "auto": an explicit source makes Interpreter skip detection
+    # and report confidence 1.0, which would defeat the acceptance gate.
+    assert captured["json"]["source"] == "auto"
 
 
 def test_translate_passthrough_unchanged(monkeypatch):