Phase 6.4.25c: correct flex.appendChild -- live verification shows it works

Live probing against Penpot 2.16.2 (repeated, on a dedicated scratch page,
cleaned up afterwards) shows board.flex.appendChild(shape) is a real
function, distinct from board.appendChild, arity 1, throws nothing, and
preserves visual insertion order -- unlike board.appendChild, which still
inserts at the front as documented. The prior "broken" claim traces to the
MCP server's own high_level_overview() text (CRITICAL / BROKEN / NEVER use),
never verified with a live throw, and possibly confused with a real doc bug
that mislabels the grid 3-arg example as board.flex.

Removes flex.appendChild from the lint's FORBIDDEN patterns and from the
gate 5 forbidden-behavior veto, updates PENPOT_API_VERIFIED.md with the
verified facts, and drops the now-obsolete pattern from the replay filters
in 05_build_dataset.py and 07_build_lora2_mix.py.

scripts/07_build_lora2_mix.py

@@ -128,7 +128,6 @@ N_PENPOT_BASICOS = 50
 FORBIDDEN_REPLAY = [
     ("findShapeById 2 args", re.compile(r"findShapeById\s*\([^)]*,")),
     (".layout inexistente", re.compile(r"\.layout\b(?!Child|Cell)")),
-    ("flex.appendChild", re.compile(r"\.flex\s*\.\s*appendChild\s*\(")),
     ("createText sin argumento", re.compile(r"createText\s*\(\s*(''|\"\")?\s*\)")),
     ("textAlign", re.compile(r"\.textAlign\s*=")),
     ("importImage/createImage/filePath", re.compile(r"\b(importImage|import_image|createImage\s*\(|filePath)\b")),