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.

lib/scheme/tests/syntax.sx

@@ -234,4 +234,55 @@
   (let ((env (scm-syn-env))) (scm-syn-eval-all "(define x 42) 'x" env))
   "x")
 
+;; ── let / let* ───────────────────────────────────────────────────
+(scm-syn-test "let: returns body"
+  (scm-syn-eval "(let ((x 5)) (+ x 1))" (scm-syn-env)) 6)
+(scm-syn-test "let: multiple bindings"
+  (scm-syn-eval "(let ((x 3) (y 4)) (+ x y))" (scm-syn-env)) 7)
+(scm-syn-test "let: parallel (RHS sees outer)"
+  (let ((env (scm-syn-env)))
+    (scm-syn-eval-all "(define x 1) (let ((x 10) (y x)) y)" env)) 1)
+(scm-syn-test "let: bindings don't leak"
+  (let ((env (scm-syn-env)))
+    (scm-syn-eval-all "(define x 1) (let ((x 99)) x) x" env)) 1)
+(scm-syn-test "let*: sequential"
+  (scm-syn-eval "(let* ((x 1) (y (+ x 1)) (z (+ y 1))) z)"
+                (scm-syn-env)) 3)
+(scm-syn-test "let*: shadow earlier"
+  (scm-syn-eval "(let* ((x 1) (x 2)) x)" (scm-syn-env)) 2)
+
+;; ── cond / when / unless ─────────────────────────────────────────
+(scm-syn-test "cond: first match"
+  (scm-syn-eval "(cond (#f 1) (#t 2) (#t 3))" (scm-syn-env)) 2)
+(scm-syn-test "cond: else"
+  (scm-syn-eval "(cond (#f 1) (else 99))" (scm-syn-env)) 99)
+(scm-syn-test "cond: untaken not evaluated"
+  (scm-syn-eval "(cond (#t 7) (nope ignored))" (scm-syn-env)) 7)
+(scm-syn-test "cond: no match returns nil"
+  (scm-syn-eval "(cond (#f 1) (#f 2))" (scm-syn-env)) nil)
+(scm-syn-test "cond: test-only clause"
+  (scm-syn-eval "(cond (42))" (scm-syn-env)) 42)
+(scm-syn-test "when: true"
+  (scm-syn-eval "(when #t 1 2 3)" (scm-syn-env)) 3)
+(scm-syn-test "when: false"
+  (scm-syn-eval "(when #f nope)" (scm-syn-env)) nil)
+(scm-syn-test "unless: false"
+  (scm-syn-eval "(unless #f 42)" (scm-syn-env)) 42)
+(scm-syn-test "unless: true"
+  (scm-syn-eval "(unless #t nope)" (scm-syn-env)) nil)
+
+;; ── and / or ─────────────────────────────────────────────────────
+(scm-syn-test "and: empty"
+  (scm-syn-eval "(and)" (scm-syn-env)) true)
+(scm-syn-test "and: all truthy returns last"
+  (scm-syn-eval "(and 1 2 3)" (scm-syn-env)) 3)
+(scm-syn-test "and: short-circuit on #f"
+  (scm-syn-eval "(and 1 #f nope)" (scm-syn-env)) false)
+(scm-syn-test "or: empty"
+  (scm-syn-eval "(or)" (scm-syn-env)) false)
+(scm-syn-test "or: first truthy"
+  (scm-syn-eval "(or #f 42 nope)" (scm-syn-env)) 42)
+(scm-syn-test "or: all #f"
+  (scm-syn-eval "(or #f #f #f)" (scm-syn-env)) false)
+
 (define scm-syn-tests-run! (fn () {:total (+ scm-syn-pass scm-syn-fail) :passed scm-syn-pass :failed scm-syn-fail :fails scm-syn-fails}))