Rewrite INTERNALS.md to match current data processing

Rewrite INTERNALS.md to match current data processing

Rewritten from scratch, much shorter and reflecting reality:
  - typecore.js as the DOM-free, unit-tested unification/matching core
    (makeTypeCore), and one-sided matchTypeArg (a concrete instance no
    longer 'matches' an abstract requirement).
  - reduceTypeArg walks the family index, which now folds in closed-family
    tfEquations.
  - deterministic merge (dumps sorted by package/module before dedup).
  - data-instance constructor extraction with rep-tyvar substitution.
  - the <-escaping of the embedded JSON.
  - typeArg/tyConArg/dataFamilyRepArg split.
Dropped the verbose prose and ToC.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>