2026-09-02 - (/loop) Packed typed-buffer SNORM load: the clamp, GPU-verified

Increment 3b of the narrow-format work (D466): SNORM, UNORM's signed twin and the first kind that clamps. A SNORM component is its signed field over the signed maximum 2^(width-1) - 1 (127 for a byte), spanning -1.0..=1.0.

• No new SPIR-V ops were needed - CONVERT_S_TO_F went in with UNORM last tick, and FORD_LESS_THAN (184) and SELECT (169) were already present with layout-table rows. (FORD_LESS_THAN was there all along; a single-line grep had missed its multi-line table entry.)
• Added Model::signed_to_float_bits, the exact twin of unsigned_to_float_bits differing only in the convert opcode - the pair exists precisely because choosing the wrong one is silent.
• The clamp, without a float-typed select. The most-negative code (-2^(width-1)) over the maximum lands a hair past -1.0 - -128/127 = -1.0079 - and the reference pins it at -1.0. So after the divide the low end is clamped: compare value < -1.0 (a float compare on the bitcast values), then pick between -1.0's bits and the value's bits. pick selects between two u32 bit patterns, which is bit-identical to selecting between the two floats, so no f32-typed OpSelect is needed - the existing u32 pick does it.
• Test a_packed_snorm_load_normalises_each_component: bytes 127, 0, -128, -127 as BUF_FMT_8_8_8_8_SNORM (code 57) must read back as the float bits of 1.0, 0.0, -1.0, -1.0, no epsilon. The third byte is the clamp case (-128/127 is past -1.0, pinned to -1.0); the fourth is the boundary that is already exactly -1.0 (-127/127) and so does not trip the strict <. A missing clamp reads the third as -1.0079 and fails. Executed on a real device.

clippy (lib and --tests) and fmt clean; spirv (15) and translate (107 execute + 13 + 2) suites green; the four earlier packed tests and the refusal gate unchanged.

All four integer/normalised kinds now translate. Next: FLOAT16 (increment 4) - a 16-bit half unpacked to f32. The clean route is GLSL UnpackHalf2x16 (ext-inst 62), which needs the OpExtInstImport "GLSL.std.450" id - check whether the builder already imports the GLSL ext set (the memory model names GLSL450 but that is not the same as an ext-inst import); if not, add the import once and thread its id through, or unpack the half manually (sign/exponent/mantissa) if the import plumbing is more than the increment warrants. Then SRGB, packed stores, multi-word widths.