asm: when/unless as always-on special forms (defect #31)

ecdsa cross-tier validation blocked on `when` and `unless` being
unbound on the asm tier. R7RS standard control forms — present on
Python (lumbda.py) and C (c/) tiers, but absent on asm because the
existing macro facility (define-macro) only ships under CL_FULL.

Path B chosen (special-form dispatch table extension):
  - sf_when / sf_unless length-prefixed symbol names
  - sym_when_val / sym_unless_val interned at init_special_forms
  - dispatch cases in .eval_top alongside .ev_and / .ev_or
  - .ev_when / .ev_unless evaluators reuse .ev_begin for the body
    branch and .ev_begin_void for the skip branch (TCO preserved)

Available on every asm tier (plain `lumbda`, `lumbda-gc`,
`lumbda-full`). Binary size impact:
  lumbda     60488 → 60768  (+280, +0.46%)
  lumbda-gc  69224 → 69496  (+272, +0.39%)
  lumbda-full 71720 → 72000 (+280, +0.39%)

All 158 asm tests still pass. Tested truthy/falsy/multi-form bodies
on all three tiers. ecdsa search.lsp now runs on asm-full with output
byte-identical to Python tier (v3-clifford-only winner, score 0).

Note: ecdsa search.lsp also depends on `case`, which is only present
under CL_FULL (carved into cl_full_prelude as a define-macro form).
That gap blocks lumbda-gc cross-tier validation and is out of scope
for this commit.

asm/lumbda.s

@@ -246,6 +246,8 @@ sf_define_mac:  .byte 12; .ascii "define-macro"
 sf_and:         .byte 3;  .ascii "and"
 sf_or:          .byte 2;  .ascii "or"
 sf_else:        .byte 4;  .ascii "else"
+sf_when:        .byte 4;  .ascii "when"
+sf_unless:      .byte 6;  .ascii "unless"
 
 # Builtin names (length-prefixed)
 bn_add:         .byte 1;  .ascii "+"
@@ -554,6 +556,8 @@ sym_define_mac_val:  .quad 0
 sym_and_val:    .quad 0
 sym_or_val:     .quad 0
 sym_else_val:   .quad 0
+sym_when_val:   .quad 0
+sym_unless_val: .quad 0
 
 # ============================================================
 .bss
@@ -2811,6 +2815,10 @@ eval:
     je .ev_and
     cmpq sym_or_val(%rip), %rbx
     je .ev_or
+    cmpq sym_when_val(%rip), %rbx
+    je .ev_when
+    cmpq sym_unless_val(%rip), %rbx
+    je .ev_unless
 .ifdef CL_FULL
     cmpq sym_quasiquote_val(%rip), %rbx
     je .ev_quasiquote
@@ -3517,6 +3525,52 @@ env_set_both:
     popq %rbx
     ret
 
+# ---- when ----
+# (when test body...) — if test is truthy, eval body as begin;
+# else return void. Available on every asm tier (defect #31 in our
+# ecdsa cross-tier validation: when/unless are R7RS standard but
+# previously only reachable on tiers carrying define-macro).
+.ev_when:
+    # %r12 = (test body...)
+    cmpq $VAL_NIL, %r12
+    je .ev_begin_void          # (when) → void (degenerate)
+    movq %r12, %rax
+    andq $-8, %rax
+    movq (%rax), %rdi          # test expr
+    movq 8(%rax), %r12         # body forms
+    pushq %r12
+    movq %rbp, %rsi
+    call eval
+    popq %r12
+    cmpq $VAL_FALSE, %rax
+    je .ev_begin_void
+    cmpq $VAL_NIL, %rax
+    je .ev_begin_void
+    # Truthy → run body via begin (TCO on last form)
+    jmp .ev_begin
+
+# ---- unless ----
+# (unless test body...) — inverse of when.
+.ev_unless:
+    cmpq $VAL_NIL, %r12
+    je .ev_begin_void          # (unless) → void
+    movq %r12, %rax
+    andq $-8, %rax
+    movq (%rax), %rdi          # test expr
+    movq 8(%rax), %r12         # body forms
+    pushq %r12
+    movq %rbp, %rsi
+    call eval
+    popq %r12
+    cmpq $VAL_FALSE, %rax
+    je .ev_unless_run
+    cmpq $VAL_NIL, %rax
+    je .ev_unless_run
+    # Truthy → skip body, return void
+    jmp .ev_begin_void
+.ev_unless_run:
+    jmp .ev_begin
+
 .ifdef CL_FULL
 # ---- quasiquote (CL_FULL only) ----
 # (quasiquote template) walks `template`:
@@ -8606,6 +8660,14 @@ init_special_forms:
     call intern_static
     movq %rax, sym_else_val(%rip)
 
+    leaq sf_when(%rip), %rdi
+    call intern_static
+    movq %rax, sym_when_val(%rip)
+
+    leaq sf_unless(%rip), %rdi
+    call intern_static
+    movq %rax, sym_unless_val(%rip)
+
     # Intern the multi-value marker symbol used by (values) and
     # (call-with-values). Stored once at init.
     leaq bn_mval_tag(%rip), %rdi