isqrt: add integer square root builtin to all three impls

Semantics: (isqrt n) → floor(sqrt(n)). Negative argument errors.
Matches Python 3.8+ math.isqrt and R7RS exact-integer-sqrt contract.

- Python: wraps math.isqrt via lambda registration
- C: hand-rolled bit-by-bit algorithm in bi_isqrt (O(log n), no FPU)
- asm: new BI_ISQRT=109, bit-by-bit algorithm in integer registers
       (%r8/%r9/%r10). Negative input → stderr + exit(1) like other
       errors. GC builtin constants bumped to 110-114.

Tests:
- tests/functional.lsp: 7 shared tests (0, 1, perfect squares, floor
  cases, large values). Python + C now 196 each (was 189).
- asm/test.sh: 5 asm-local tests. asm suite now 142 (was 137).

MOAD: all three implementations O(log n), no O(N²) hazards.

asm/lumbda.s

@@ -194,15 +194,16 @@
 .equ BI_HS_SIZE,  106
 .equ BI_HS_LIST,  107
 .equ BI_TCPSENDFILE, 108
+.equ BI_ISQRT,    109
 .ifdef GC_NAIVE
-.equ BI_GC_COLLECT, 109
-.equ BI_GC_STATS,   110
-.equ BI_WITH_ARENA, 111
-.equ BI_ARENA_STATS, 112
-.equ BI_ARENA_SET_MODE, 113
-.equ BI_COUNT,      114
+.equ BI_GC_COLLECT, 110
+.equ BI_GC_STATS,   111
+.equ BI_WITH_ARENA, 112
+.equ BI_ARENA_STATS, 113
+.equ BI_ARENA_SET_MODE, 114
+.equ BI_COUNT,      115
 .else
-.equ BI_COUNT,    109
+.equ BI_COUNT,    110
 .endif
 
 # ============================================================
@@ -299,6 +300,7 @@ bn_listp:       .byte 5;  .ascii "list?"
 bn_substr:      .byte 9;  .ascii "substring"
 bn_expt:        .byte 4;  .ascii "expt"
 bn_gcd:         .byte 3;  .ascii "gcd"
+bn_isqrt:       .byte 5;  .ascii "isqrt"
 bn_integerp:    .byte 8;  .ascii "integer?"
 bn_portalsave:  .byte 11; .ascii "portal-save"
 bn_portalresume:.byte 13; .ascii "portal-resume"
@@ -388,6 +390,7 @@ bi_names:
     .quad bn_htexists, bn_htsize, bn_htkeys, bn_htvals, bn_htalist
     .quad bn_hsmake, bn_hsp, bn_hsadd, bn_hshas, bn_hssize, bn_hslist
     .quad bn_tcpsendfile
+    .quad bn_isqrt
 .ifdef GC_NAIVE
     .quad bn_gccollect, bn_gcstats, bn_witharena, bn_arenastats, bn_arenamode
 .endif
@@ -399,6 +402,8 @@ err_notproc:    .ascii "Error: not a procedure\n"
 .equ err_notproc_len, . - err_notproc
 err_oom:        .ascii "Error: out of memory\n"
 .equ err_oom_len, . - err_oom
+err_isqrt_neg:  .ascii "Error: isqrt: negative argument\n"
+.equ err_isqrt_neg_len, . - err_isqrt_neg
 
 # Print strings
 s_true:         .ascii "#t"
@@ -3355,6 +3360,8 @@ eval_list:
     je bi_expt
     cmpq $BI_GCD, %rax
     je bi_gcd
+    cmpq $BI_ISQRT, %rax
+    je bi_isqrt
     cmpq $BI_INTEGERP, %rax
     je bi_integerp
     cmpq $BI_PORTALSAVE, %rax
@@ -4663,6 +4670,58 @@ bi_gcd:
     call make_int
     RET_VAL
 
+# bi_isqrt: floor of integer square root (bit-by-bit, no FPU).
+# Negative input -> error exit. Registers: %r8 = x, %r9 = res, %r10 = bit.
+bi_isqrt:
+    GETARG %rax
+    sarq $3, %rax                # untag
+    testq %rax, %rax
+    js .bisqrt_neg
+    cmpq $2, %rax
+    jl .bisqrt_small             # v < 2 -> result is v itself
+    movq %rax, %r8               # x
+    xorq %r9, %r9                # res
+    movq $1, %r10
+    shlq $62, %r10               # bit = 1<<62
+.bisqrt_align:
+    cmpq %r8, %r10
+    jbe .bisqrt_loop
+    shrq $2, %r10
+    jmp .bisqrt_align
+.bisqrt_loop:
+    testq %r10, %r10
+    jz .bisqrt_ret
+    movq %r9, %rcx
+    addq %r10, %rcx              # rcx = res + bit
+    cmpq %rcx, %r8
+    jb .bisqrt_no
+    subq %rcx, %r8               # x -= res + bit
+    shrq $1, %r9
+    addq %r10, %r9               # res = (res>>1) + bit
+    jmp .bisqrt_next
+.bisqrt_no:
+    shrq $1, %r9                 # res >>= 1
+.bisqrt_next:
+    shrq $2, %r10                # bit >>= 2
+    jmp .bisqrt_loop
+.bisqrt_ret:
+    movq %r9, %rdi
+    call make_int
+    RET_VAL
+.bisqrt_small:
+    movq %rax, %rdi
+    call make_int
+    RET_VAL
+.bisqrt_neg:
+    movq $SYS_WRITE, %rax
+    movq $2, %rdi
+    leaq err_isqrt_neg(%rip), %rsi
+    movq $err_isqrt_neg_len, %rdx
+    syscall
+    movq $SYS_EXIT, %rax
+    movq $1, %rdi
+    syscall
+
 bi_vector:
     # (vector e1 e2 ...) — build from remaining args in %r12
     # Count args

asm/test.sh

@@ -52,6 +52,11 @@ check "max"        "(max 3 1 4)"       "4"
 check "expt"       "(* 2 2 2 2 2 2 2 2 2 2)"  "1024"
 check "div"        "(/ 10 2)"           "5"
 check "expt-bi"    "(expt 2 10)"        "1024"
+check "isqrt-0"    "(isqrt 0)"          "0"
+check "isqrt-1"    "(isqrt 1)"          "1"
+check "isqrt-perfect" "(isqrt 144)"     "12"
+check "isqrt-floor" "(isqrt 10)"        "3"
+check "isqrt-big"  "(isqrt 1000000000000)" "1000000"
 check "odd?"       "(odd? 3)"           "#t"
 check "odd?-even"  "(odd? 4)"           "#f"
 check "even?"      "(even? 4)"          "#t"