Fix riscv64 hosted acceptance-test doc; port 3 clang-surfaced bugs from punch list

Addresses items #1 (partial) and #2 of
docs/working/archive/session-logs/2026-07-24-punch-list.md.

docs/lithosananke/hosted-acceptance-test/README.md:
- riscv64 leg used riscv64-linux-gnu-gcc, which fails to build this tree
  (nanosleep visibility under -std=c99). Replaced with the working
  clang-18 --target=riscv64-linux-gnu --sysroot=/usr/riscv64-linux-gnu
  invocation, verified end-to-end.
- All three arch sections referenced a -c "<script>" flag that has never
  existed in cli.c/main.c. Corrected to the working
  `echo "..." | starforth -s` pattern, verified on all three architectures.
- Updated Prerequisites: qemu-user alone is sufficient (guest binaries are
  static; qemu-user-static provides static *emulators*, not required here).

Source fixes (ported from the old pre-split monorepo's master, commit
4db9946a, where they were made but never carried over to this line):
- src/math_portable.c: `-100LL << 16` is UB (shifting a negative value)
  under clang's -Wshift-negative-value; changed to `-(100LL << 16)`.
- src/physics_pipelining_metrics.c: removed dead q48_mul_q48()
  (-Wunused-function under clang; GCC doesn't flag this by default).
- src/word_source/editor_words.c: removed dead set_scr() (same reason).

These three were required just to get the documented clang build to
compile at all. The punch list's higher-severity item — a genuine
SIGSEGV-causing register-reuse hazard in vm_pop_asm/vm_rpop_asm
(include/vm_asm_opt_riscv64.h) — is intentionally NOT included here; it's
a separate, more careful change and isn't required for this build to
succeed (latent only under clang; this repo's kernel build uses GCC).

Verified: all three hosted builds compile and run correctly (amd64
native, aarch64 via qemu-aarch64, riscv64 via qemu-riscv64), each
printing "3 Goodbye!" for the piped `1 2 + . BYE` script. All three
Makefile.starkernel builds (amd64/aarch64/riscv64) still compile cleanly
with these shared vendored-source changes.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/math_portable.c

@@ -94,7 +94,7 @@ static int64_t exp_q48(int64_t x) {
     /* Handle extremes */
     if (x == 0) return Q48;
     if (x > (20LL << 16)) return Q48 << 8;  /* Approximate e^20 */
-    if (x < (-100LL << 16)) return 0;       /* Underflow to zero */
+    if (x < -(100LL << 16)) return 0;       /* Underflow to zero */
 
     /* Taylor series: exp(x) = Σ x^n / n! (in fixed-point) */
     int64_t result = Q48;  /* Start with 1 */