docs: track untyped-JS / no-static-type-checking as tech debt

[ohxyz]

What

Adds a new [low] tech-debt entry to docs/TECH_DEBT.md: "Source is untyped JavaScript — no static type checking."

Placed just above the existing "bwin publishes no TypeScript types" entry, with cross-links both ways.

Why

The two are related but distinct, and the existing entry only covered one side:

• Existing entry — bwin ships no .d.ts, so consumers hand-maintain a declare module 'bwin' shim (output gap).
• New entry — bwin's own source has no compiler in the loop: no tsconfig, no checkJs, no JSDoc types. The shapes that matter most (sash.store keys, the config→sash compile output, the actions undefined-vs-null contract, the public API) are enforced only at runtime and documented only in prose.

Notes

• Impact is tied to concrete at-risk refactors: the queued bw-pot/bw-glass-action rename and the in-flight drag-and-drop rework, plus drift against the react-bwin shim.
• Fix direction recommends an incremental tsconfig + allowJs/checkJs + JSDoc path (starting at the public API / store/config/sash shapes) over a big-bang .ts conversion — and notes a checkJs setup that emits .d.ts would subsume the entry below it.

Docs-only change. Branched from main (TECH_DEBT.md is identical there), independent of refactor/drag-and-drop.