wasm: cap the stack at 256KB - 16.25MB -> 0.5MB per Game BoyHEADmaster

zig defaults the wasm stack to 16MB, and that lands in the module's *initial*
memory, so every instance allocated 16.25MB before executing an instruction.
With several Game Boys on one page that was the dominant cost: 123MB for six.
The machine is a static and nothing here recurses deeply, so 256KB is plenty.

Per instance: 20.5MB -> 4.8MB of linear memory (six: 123MB -> 29MB). Note the
spelling - zig cc rejects -Wl,--stack-size and -zstack-size, and wants
-Wl,-z,stack-size=N.

With memory no longer the constraint, the page cap goes 6 -> 8 on measurement
rather than guesswork. Emulation-only, one thread: 12.5x realtime at 1, 3.1x
at 4, 1.5x at 8, and exactly 1.0x at 12 - i.e. ~12 is where a thread runs out
of road with nothing left for rendering. 8 holds a verified 60fps each in a
real browser with all eight leased.

Golden frames are still pixel-exact on cgb-acid2, dmg-acid2, cpu_instrs@600
and pky@400 with its .sav, and two Game Boys still lease and ping 4/4.

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