portal-rng: add (random-seed-from-os!) across all three tiers

Ticket 0002 — reads 8 bytes from /dev/urandom (little-endian u64) and
seeds xoshiro256**. Opt-in kernel entropy for stochastic runs; the
default stays deterministic (k=0 at startup), so ticket 0001's
portal-reproducibility contract is unchanged.

Real-world flow now one call away:
  Machine A: (random-seed-from-os!) + run simulation + portal-save
  Machine B: portal-resume — same stream, bit-for-bit

All three impls fail loud on /dev/urandom trouble (LispErr in Python
and C, stderr + exit(1) in asm) — no silent fallback to a weak seed.

Tests:
- tests/functional.lsp: 2 new shared asserts (entropic + replay)
- asm/test.sh: 2 new asm-local checks (149 total, was 147)
- make test-all green across Python (205), C (205), asm (149)

Whitepaper §7.5 gains one sentence noting the OS-seed path.
unmoad: zero new findings in added code.

asm/lumbda.s

@@ -199,15 +199,16 @@
 .equ BI_RANDOMINT,      111
 .equ BI_RANDOMSTATE,    112
 .equ BI_RANDOMSTATESET, 113
+.equ BI_RANDOMSEEDFROMOS, 114
 .ifdef GC_NAIVE
-.equ BI_GC_COLLECT, 114
-.equ BI_GC_STATS,   115
-.equ BI_WITH_ARENA, 116
-.equ BI_ARENA_STATS, 117
-.equ BI_ARENA_SET_MODE, 118
-.equ BI_COUNT,      119
+.equ BI_GC_COLLECT, 115
+.equ BI_GC_STATS,   116
+.equ BI_WITH_ARENA, 117
+.equ BI_ARENA_STATS, 118
+.equ BI_ARENA_SET_MODE, 119
+.equ BI_COUNT,      120
 .else
-.equ BI_COUNT,    114
+.equ BI_COUNT,    115
 .endif
 
 # ============================================================
@@ -309,6 +310,7 @@ bn_randomseed:  .byte 12; .ascii "random-seed!"
 bn_randomint:   .byte 10; .ascii "random-int"
 bn_randomstate: .byte 12; .ascii "random-state"
 bn_randomstateset: .byte 13; .ascii "random-state!"
+bn_randomseedfromos: .byte 20; .ascii "random-seed-from-os!"
 bn_integerp:    .byte 8;  .ascii "integer?"
 bn_portalsave:  .byte 11; .ascii "portal-save"
 bn_portalresume:.byte 13; .ascii "portal-resume"
@@ -404,6 +406,7 @@ bi_names:
     .quad bn_tcpsendfile
     .quad bn_isqrt
     .quad bn_randomseed, bn_randomint, bn_randomstate, bn_randomstateset
+    .quad bn_randomseedfromos
 .ifdef GC_NAIVE
     .quad bn_gccollect, bn_gcstats, bn_witharena, bn_arenastats, bn_arenamode
 .endif
@@ -421,6 +424,9 @@ err_rng_bad_n:  .ascii "Error: random-int: n must be positive\n"
 .equ err_rng_bad_n_len, . - err_rng_bad_n
 err_rng_short: .ascii "Error: random-state!: expected list of 8 integers\n"
 .equ err_rng_short_len, . - err_rng_short
+err_rng_urandom: .ascii "Error: random-seed-from-os!: /dev/urandom unavailable\n"
+.equ err_rng_urandom_len, . - err_rng_urandom
+s_dev_urandom:  .asciz "/dev/urandom"
 
 # Print strings
 s_true:         .ascii "#t"
@@ -3397,6 +3403,8 @@ eval_list:
     je bi_random_state
     cmpq $BI_RANDOMSTATESET, %rax
     je bi_random_state_bang
+    cmpq $BI_RANDOMSEEDFROMOS, %rax
+    je bi_random_seed_from_os
     cmpq $BI_INTEGERP, %rax
     je bi_integerp
     cmpq $BI_PORTALSAVE, %rax
@@ -4953,6 +4961,53 @@ bi_random_state_bang:
     movq $VAL_VOID, %rax
     RET_VAL
 
+# bi_random_seed_from_os: (random-seed-from-os!) -> void
+# Reads 8 bytes from /dev/urandom, interprets as little-endian u64,
+# seeds xoshiro256**. Opt-in entropy for stochastic runs; determinism
+# remains the default. See docs/tickets/0002-os-entropy-seed.md.
+bi_random_seed_from_os:
+    # open("/dev/urandom", O_RDONLY, 0)
+    movq $SYS_OPEN, %rax
+    leaq s_dev_urandom(%rip), %rdi
+    movq $O_RDONLY, %rsi
+    xorq %rdx, %rdx
+    syscall
+    testq %rax, %rax
+    js .brsfo_fail
+    movq %rax, %rbx                 # fd
+    # read 8 bytes into stack slot
+    subq $8, %rsp
+    movq $SYS_READ, %rax
+    movq %rbx, %rdi
+    movq %rsp, %rsi
+    movq $8, %rdx
+    syscall
+    cmpq $8, %rax
+    jne .brsfo_short
+    movq (%rsp), %rdi               # seed = little-endian u64
+    addq $8, %rsp
+    pushq %rbx                      # save fd across rng_seed call
+    call rng_seed
+    popq %rbx
+    # close(fd)
+    movq $SYS_CLOSE, %rax
+    movq %rbx, %rdi
+    syscall
+    movq $VAL_VOID, %rax
+    RET_VAL
+.brsfo_short:
+    addq $8, %rsp
+    # fall through to fail with same error (urandom unavailable / short)
+.brsfo_fail:
+    movq $SYS_WRITE, %rax
+    movq $2, %rdi
+    leaq err_rng_urandom(%rip), %rsi
+    movq $err_rng_urandom_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

@@ -62,6 +62,8 @@ check "random-int-2"    "(begin (random-seed! 42) (random-int 1000000) (random-i
 check "random-reseed"   "(begin (random-seed! 42) (random-int 1000000) (random-seed! 42) (random-int 1000000))" "558742"
 check "random-state-len" "(begin (random-seed! 42) (length (random-state)))" "8"
 check "random-state-restore" "(begin (random-seed! 42) (define s (random-state)) (random-int 1000000) (random-state! s) (random-int 1000000))" "558742"
+check "random-os-entropic" "(begin (random-seed-from-os!) (define a (random-state)) (random-seed-from-os!) (not (equal? a (random-state))))" "#t"
+check "random-os-replay" "(begin (random-seed-from-os!) (define s (random-state)) (define v (random-int 1000000)) (random-state! s) (= v (random-int 1000000)))" "#t"
 check "odd?"       "(odd? 3)"           "#t"
 check "odd?-even"  "(odd? 4)"           "#f"
 check "even?"      "(even? 4)"          "#t"