feat: TECHNICAL_PLACEHOLDER (NCM 00000000) is replaceable by the product enrichment; real values preserved; Agent 1.3.4

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

central/app/enrichment/placeholders.py

@@ -0,0 +1,27 @@
+"""TECHNICAL_PLACEHOLDER semantics for the product enrichment.
+
+The SGV refuses to save a product without an NCM, so operators type a technical filler to get past the form. That filler is NOT a human fiscal decision: it means "not classified yet".
+Explicit allowlist PER FIELD (never generalised: "0", "99999999", "SEM NCM" ... are NOT placeholders until the real SGV behaviour is confirmed). The Agent carries the same table
+(agent/src/gti_fiscal_agent/enrichment.py PLACEHOLDERS); a test asserts both are identical.
+"""
+import re
+
+PRODUCT_ENRICHMENT_PLACEHOLDERS = {"CLASSIFICACAO_FISCAL": frozenset({"00000000"})}
+
+
+def _norm(field: str, value) -> str:
+    text = "" if value is None else str(value).strip()
+    return re.sub(r"\D", "", text) if field == "CLASSIFICACAO_FISCAL" else text     # 0000.00.00 and 00000000 are the same NCM filler
+
+
+def is_placeholder(field: str, value) -> bool:
+    return _norm(field, value) in PRODUCT_ENRICHMENT_PLACEHOLDERS.get(field, frozenset())
+
+
+def is_blank(value) -> bool:
+    return not ("" if value is None else str(value)).strip()
+
+
+def is_writable_empty(field: str, value) -> bool:
+    """True when the field is empty OR holds the approved technical placeholder. A real value (even a wrong one) is never writable here."""
+    return is_blank(value) or is_placeholder(field, value)