refactor(math): extract core base class and de-duplicate math module (#47)

[kalwalt]

First de-duplication step of #47 — the proof-of-pattern PR that every subsequent module extraction will follow. Guarded by the 57-test parity suite (#39/#49).

What changed

New src/core/core.ts — the base class, extracted

The base jsfeatNext class (constants, per-instance cache/data-type helpers, static module slots) moves out of the monolith verbatim. Why it must be its own file: modules extend jsfeatNext (that's the public contract — instance methods like get_data_type() and static inheritance like jsfeatNext.math.EPSILON depend on it), and a module can't extend a class defined in the same file that imports the module back — that's a circular ESM import that throws at load time. The one-way flow is now:

core ← modules (math, …) ← jsfeatNext.ts (aggregator) ← index.ts

This is the audit's §4 target architecture starting to materialize (see also #40).

src/math/math.ts — stub replaced by the real implementation

The type-only stub (every method throw new Error("Method not implemented.")) is gone; the inline implementation from the monolith moved in verbatim (get_gaussian_kernel, deprecated perspective_4point_transform, qsort, median). Extraction was done mechanically (sed line-range copy), not retyped.

One deliberate signature fix: median() now accepts Int32Array | Float32Array as well as number[] — motion_estimator.lmeds always called it with a Float32Array cache buffer at runtime; the old stub hid that behind any. Types only, zero behavior change.

src/jsfeatNext.ts — shrinks by ~560 lines

Imports the base from core, attaches math from its module. On its way to becoming a thin aggregator.

Known-temporary (tightened as #47 proceeds)

The affine2d/homography2d static slots on the base are typed any until those classes are extracted from the monolith in a later PR.

Verification (behavior-preserving, proven)

• tsc --noEmit → clean
• npm test → 57/57 parity tests pass
• UMD build verified: require shape ({ jsfeatNext }), new jsfeatNext.math() instanceof jsfeatNext → true, static-constant inheritance (jsfeatNext.math.EPSILON === jsfeatNext.EPSILON) → true, gaussian-kernel numerics identical
• dist//types/ deliberately untouched (rebuilt at release time, per fix(motion_estimator): restore affine2d error() and check_subset() (#51) #52 precedent)

Next modules in the queue (same pattern)

imgproc → fast_corners → pyramid_t → linalg → orb → yape06 → motion_estimator (+ affine2d/homography2d/motion_model) → optical_flow_lk

Advances #47.

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