Refactor: target architecture (thin aggregator, one class per module, shared cache, static facade)

[kalwalt]

Adopt the recommended architecture: src/jsfeatNext.ts becomes a thin aggregator; one real class per module; a shared singleton cache (drop the per-instance allocation and the God-object base class every module extends); and a static-facade API to restore jsfeat drop-in parity. See docs/jsfeat-parity-and-refactor-audit.md (Axis 4).

[kalwalt]

Partial progress via #47 (shipped in 0.8.0):

• ✅ src/jsfeatNext.ts is now a thin aggregator (59 lines)
• ✅ One real class per module (no more type-only stubs)

Still open:

• ⬜ Shared singleton cache (currently still per-instance — each new jsfeatNext.<module>() allocates its own cache pool via the base class)
• ⬜ Static-facade API for jsfeat drop-in parity

Both remaining items overlap with #41's scope (per-instance cache and static-vs-instance are literally in that issue's title) — recommend tackling them together as part of #41's breaking-change work rather than as a separate pass.