A JS/WASM sandbox for halfphi

[isenbek]

Asked for when the library was scoped, and not built yet: a browser surface that
drives halfphi directly rather than through the 6502 layer, so the library can
be used and demonstrated on any die in the visual6502 three-file form.

What it needs to do

Load a chip's segdefs.js / transdefs.js / nodenames.js at runtime —
not baked in at build time, which is how this project's wasm works today and is
the reason it is about one chip.

That is also the licence-safe shape: the sandbox ships no die data, the user
supplies it, and the CC BY-NC-SA obligations never attach to what is served.

Rough surface

• Parse three files into a netlist; report nodes, transistors, names, polygons,
  layers present, rails.
• Power on, settle, report convergence.
• Read and drive named nodes.
• Ideally: draw the polygons, since Parsed already returns them and the
  renderer in web/ proves the geometry pipeline works.

Watch out for

• The layer set differs per chip: the 6800 and Z80 have no layer 2, so the
  exploded view's layer collapsing cannot be a constant here.
• Rails differ: the 6800 calls ground gnd.
• The Z80 does not converge from a cold power-on (The Z80 does not converge from a cold power-on halfphi#2), so
  the sandbox must display that state rather than spin.