model_compile threw OpenSCAD's stderr away, so a valid model and one that
silently produced an empty or broken mesh looked identical to the caller. The
actionable part — ECHO values, the Top level object/Facets block, CGAL
warnings — is now kept (cache and timing noise filtered, capped at 4000 chars)
and returned in both `output` and `metadata.diagnostics`.

scad_analyze flagged real OpenSCAD builtins (assert, children, each,
linear_extrude, minkowski, text, render, …) as undefined identifiers, and its
for-loop regex only matched `for` at a word boundary so `intersection_for`
tripped it. Both fixed; render_stl.md spells out what a PNG does and does not
confirm now that the frame is autocenter/viewall normalized.