spec: thermite-spec design doc + combinator oracle (#2)

Loop-3 prerequisites for the thermite-spec build, committed before the
builder so the build references tracked files (a loop-2 lesson):

- .design/spec/spectherm-combinators.md — the registry + SpecTherm validator
  contract (8 frozen combinators; validator enforces §4.2 over AST contract
  positions, recursion-bounded; SMT triggers + Verus/L1 forms deferred to #4).
- tests/golden/combinators/ — the hand-derived oracle (registry.json + 9
  accept + 4 reject cases), acto-critic-verified NO DIVERGENCE: registry
  re-derived from §4.2, every accept/reject snippet parse-checked against the
  real thermite-syntax parser (rejects are genuine semantic violations).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dollspace-gay