giles 0fccd1b353 scheme: Phase 3.5 — let/let*/cond/when/unless/and/or + 21 tests ...

Adds the rest of the standard syntactic operators, all built on the
existing eval/closure infrastructure from Phase 3:

- let — parallel bindings in fresh child env; values evaluated in
  outer env (RHS sees pre-let bindings only). Multi-body via
  scheme-eval-body.
- let* — sequential bindings, each in a nested child env; later
  bindings see earlier ones.
- cond — clauses walked in order; first truthy test wins. `else`
  symbol is the catch-all. Test-only clauses (no body) return the
  test value. Scheme truthiness: only #f is false.
- when / unless — single-test conditional execution, multi-body
  body via scheme-eval-body.
- and / or — short-circuit boolean. Empty `(and)` = true,
  `(or)` = false. Both return the actual value at the point
  of short-circuit (not coerced to bool), matching R7RS.

130 total Scheme tests (62 parse + 23 eval + 45 syntax). The
Scheme port is now self-hosting enough to write any non-stdlib
program — factorial, list operations via primitives, closures
with mutable state, all working.

Next phase: standard env (runtime.sx) with variadic +/-, list
ops as Scheme-visible applicatives.

2026-05-13 20:04:44 +00:00

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