Add hardware A/B and native Thumb test framework

[Eric Anderson (humanapp)]

Adds a framework for testing compiled programs, exercised offline and on actual microbit hardware. The motivation: validating the codegen optimizations in #11336 and #11339, so the initial test suite covers exactly what they change: condition lowering and interface dispatch. The framework itself though is general. Once this gets merged, I will rebase those PRs onto it and they'll gain real test coverage.

Testing on microbit

tests/hw-ab/ builds candidate and reference hexes, flashes a microbit, captures serial, and diffs the traces -- plus a soak mode that watches control.gcStats() for leaks. One command per case:

npm run hwab -- ab truthiness

Writing the tests caught a pre-existing compiler bug

Default parameter values are not applied on dynamic dispatch -- m(a: number, b = 5) called as x.m(1) through an interface- or any-typed reference yields NaN (the arity wrapper fills undefined; the initializer never runs). A ready-made red test is checked in disabled: uncomment the REPRO block in 57defaultparamdispatch.ts and gulp testlang fails with the assertion id. Example of code that breaks:

interface Shape {
    scaled(x: number, factor?: number): number;
}

class Thing implements Shape {
    scaled(x: number, factor = 5) {
        return x * factor;
    }
}

const t = new Thing();
const s: Shape = t;

console.log(t.scaled(2));   // 10  -- static call, default applied
console.log(s.scaled(2));   // NaN -- same object via interface: default not applied

Repros in both JS and on hardware.

(Just to make sure, I verified this bug existed prior to any of my compiler optimizations.)

---

The test framework's structure

The three layers

layer	run	catches
JS-executed semantics	gulp testlang	wrong answers -- 3 new lang-test0 files covering condition truthiness/lowering and interface dispatch, run on the simulator backend
native assembly shape	gulp testthumb	which sequences codegen chose, unencodable instructions, size swings -- compiles to ARM Thumb
on-device A/B + soak	npm run hwab	runtime failures on device: GC, panics, leaks over time

Coverage map (failure mode -> test -> layer): see tests/compile-test/lang-test0/README-codegen.md.

microbit test fixtures in pxt repo? why?

The thumb tests have to compile against some hardware target; microbit is the most natural one. tests/thumb-test/fixtures/microbit-mbcodal.json.gz snapshots a real mbcodal build: compile config, runtime hex + function table, core TS sources. It is inert input data (not pre-compiled; the current compiler always runs against it), it fails loudly if a test references a missing API, and regeneration is one documented command. Details in tests/thumb-test/README.md.

cc: Thomas Ball (@thomasjball)