nx_step_write.nx

nx_step_write.nx -- STEP WRITE (cadtwin P3a write half, first rung): emit a valid ISO-10303-21 file from a twin mesh -- FACETED B-rep class (FACETED_BREP + CLOSED_SHELL + FACE/FACE_OUTER_BOUND/POLY_LOOP + shared CARTESIAN_POINTs) with the full product-identity chain (PRODUCT->PDF->PD->PDS->SDR) and DECLARED mm units + uncertainty (the anti-JPL doctrine: our files SAY their units). Coordinates are written as EXACT decimals: fx256 -> "N.XXXXXXXX" (1/256 = 0.00390625 -> 8 fractional digits, lossless) so a round-trip through our own reader is INTEGER-EXACT by construction. Includes the matching FACETED READ path (POLY_LOOP -> triangles) -- the interop pair in one organ. HONEST scope: faceted geometry write (the mesh-twin class); analytic/NURBS B-rep WRITE = the named residual for full parity. NOTE: '#' cannot appear in NishiLang string literals (scanner gotcha) -- every STEP ref emits byte 35 via swb_c. license_tier: ORIGINAL