websim: extend the web simulator to all logue-SDK platforms

[wilvk]

Summary

Extends the existing web simulator so that every logue-SDK platform can build its DSP to
WebAssembly and run in the browser, not just NTS-1 mkII and NTS-3.

Today websim/ supports the two gen-2 NTS platforms via a wasm: recipe duplicated inline in
each project Makefile. This PR factors that recipe into a shared build fragment and adds the
host-side bridges needed by the other platforms, so the same unit.cc that ships to hardware
also runs in a browser tab for microKORG2, drumlogue, and the gen-1 targets
(prologue / minilogue xd / NTS-1 mkI).

Nothing about the hardware build changes: no linker scripts, no CMSIS, no _unit_base.c, and
no ARM build flags are touched. The wasm path is additive.

What's here

Area	Change
websim/wasm.mk	Shared wasm / wasm-build / render targets, extracted from the per-project recipe that was duplicated in every NTS project Makefile
websim/legacy.mk	The same entry point for gen-1 units, whose DSP is free C functions emitting q31 rather than a C++ class
websim/dsp/**	Host stand-ins for the firmware ROM, plus a bridge per platform family (microKORG2, drumlogue, gen-1)
platform/**	A small wasm.cc shim and a wasm target for each unit
Makefile (new, repo root)	make websim PROJECT=… to build and open one unit; make list, make clean, make setup
WEBSIM.md	Full guide: prerequisites, per-platform setup, how the bridges work, troubleshooting

Existing NTS projects

The 8 NTS-1 mkII / NTS-3 project Makefiles lose their inline wasm: block in favour of

WEBSIM_SHELL    := osc.html
WEBSIM_INCFLAGS := $(INCDIR)
include $(SANDBOXDIR)/wasm.mk

Same emcc invocation, same output — one copy instead of eight.

Bug fixes in platform/microkorg2/common/utils/

Building microKORG2 units without NEON exercised the #else branches of the SIMD helpers,
which turn out not to compile as written. These are fixed here because the wasm build needs
them, but they are latent bugs in the non-NEON path regardless of websim:

• int_simd.h — six *_str macros are missing a statement terminator:
  do { *(int32x4_t *)(ptr) = (v) } while (0);. Any use is a syntax error.
  Affects s32x4_str, s32x2x2_str, s32x4x2_str and the u32 equivalents.
• int_simd.h — const int2x2_t tmp = … in the non-NEON clip helper names a type that
  does not exist; it should be int32x2_t.
• float_simd.h — the non-NEON vtrn / vzip / vuzp helpers declare
  const float32x2x2_t v; and then assign to v.val[0], which is an assignment to a const
  object. Also switches the {{…}} aggregate initialisers to the vector constructors.

These are worth reviewing on their own merits — happy to split them into a separate PR if you
would prefer to take them independently of the websim work.

The other two files in that directory are guarded so the ARM build is byte-identical:

• cortexa7_intrinsics.h — the Cortex-A7 DSP ops are inline ARM assembly that clang cannot
  assemble for wasm. Portable equivalents with matching saturating semantics are provided under
  #ifdef __EMSCRIPTEN__. One caveat: SEL depends on the APSR.GE flags set by a preceding
  SIMD instruction, which cannot be reproduced standalone, so the shim returns its first
  operand — the same compromise as the existing gen-1 __SEL shim.
• mk2_utils.h — __builtin_prefetch requires constant arguments under clang/wasm, so
  PrefetchMemory becomes a no-op under #ifdef __EMSCRIPTEN__.

Example units

Three new units, included because they exercise build paths the existing templates don't:

• platform/drumlogue/sample-voice (SmplVox) — a sample-playback drum voice; the first
  drumlogue unit that uses the sample bank API.
• platform/nutekt-digital/twosaw — a gen-1 oscillator that derives its websim parameter
  table from manifest.json rather than a hand-written macro.
• platform/nutekt-digital/tremolo — a gen-1 modfx example.

Happy to drop these from this PR if you'd rather keep it to the build system alone.

Verification

Both gates run from a clean checkout with a pinned Emscripten (4.0.16):

$ make build-all
build-all: 28 project(s) built OK

$ make render-all
render-all: all renders finite and OK

build-all compiles and links every wasm-capable unit across all six platforms.
render-all builds each unit in a headless render mode (no AudioWorklet), runs it under node,
and asserts the output WAV is finite — catching crashes and numerical blow-ups without a
browser. Both are wired into .github/workflows/websim.yml.

Browser behaviour was checked by hand per unit: page loads cross-origin-isolated, the keyboard
drives pitch for oscillators, input plays through for effects, and every parameter slider
audibly changes the sound.

On the CI workflow: if you would rather not take a GitHub Actions workflow from an outside
contributor, say so and I'll drop .github/workflows/websim.yml — the build-all and
render-all targets stand on their own and can be run manually or wired up however you prefer.

Notes for review

• Emscripten 3.1.x or newer is required (SIMDe arm_neon.h for drumlogue, and recent
  enough to reject -I …/emscripten/system/include). The workflow pins 4.0.16.
• The NEON-heavy microKORG2 units (vox, MorphEQ, MultitapDelay, Vibrato, breveR)
  build through a struct-shape SIMD shim rather than SIMDe; see WEBSIM.md.
• Audio is not bit-exact against the ARM build — the wasm path scalarises NEON, so the
  render checks are tolerance-based rather than golden-file comparisons.
• microKORG2 oscillators are single-voice by default; vox opts into 4-voice polyphony via
  MK2_OSC_VOICES.

[radfaraf]

This is an incredible contribution. Hopefully the KORG team can find the time to test and approve this as it would greatly increase the value of their products.

[xiashj24]

Thank you for putting this together. I will take some time to review it.