Add riscv64-clang Makefile target; closes out 2026-07-24 punch list

Resolves item #3 of docs/working/archive/session-logs/2026-07-24-punch-list.md
("riscv64 hosted build isn't reachable via plain make"), the last open
item from that list. Decided against options (b) chasing GCC's riscv64
nanosleep-visibility failure at its root (undiagnosed, open-ended) and
(c) leaving it manual — instead wired the already-verified clang recipe
(commit 4485c38 / e287334) into the Makefile, mirroring the existing
rpi4-cross pattern.

Makefile: new riscv64-clang target. CFLAGS deliberately does not reuse
$(BASE_CFLAGS) (hardcodes -std=c99); clang needs -std=c11 -pthread here
instead. Registered in `make help` and .PHONY.

docs/lithosananke/hosted-acceptance-test/README.md: riscv64 section now
points at `make riscv64-clang` instead of the long manual invocation.
Updated Background section and commit list to reflect that all three
punch-list items touching this doc (#1 asm fix, #2 doc command, #3 make
target) are now resolved.

Verified: `make riscv64-clang` produces a binary with identical results
to the manual command it replaces (965 passed / 0 failed, "ALL
IMPLEMENTED TESTS PASSED!", "3 Goodbye!" for the piped acceptance script).

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

Makefile

@@ -475,7 +475,7 @@ CONFDIR = $(PREFIX)/etc/starforth
 
 .PHONY: all banner help clean clean-obj clean-docs starkernel mkcapsule capsule-gen capsule-manifest
 .PHONY: fastest fast turbo
-.PHONY: rpi4 rpi4-cross rpi4-fastest
+.PHONY: rpi4 rpi4-cross rpi4-fastest riscv64-clang
 .PHONY: minimal debug profile performance
 .PHONY: test bench benchmark
 .PHONY: asm sbom
@@ -562,6 +562,23 @@ rpi4-fastest:
 	        all
 	@echo "✓ Raspberry Pi FASTEST build ready: $(call binary_path,fastest,raspi)"
 
+# RISC-V 64 - cross-compile via clang (GCC fails to build this tree under
+# -std=c99: a nanosleep visibility failure, not yet root-caused — see
+# docs/lithosananke/hosted-acceptance-test/README.md and
+# docs/working/archive/session-logs/2026-07-24-punch-list.md item #3).
+# CFLAGS deliberately does NOT reuse $(BASE_CFLAGS): BASE_CFLAGS hardcodes
+# -std=c99, and clang needs -std=c11 -pthread here. This mirrors the exact
+# recipe verified in the hosted-acceptance-test doc, kept in sync with it.
+riscv64-clang:
+	@echo "🦀 Cross-compiling for RISC-V 64 (clang)..."
+	@$(MAKE) ARCH=riscv64 TARGET=fastest \
+	        CC="clang-18 --target=riscv64-linux-gnu --sysroot=/usr/riscv64-linux-gnu" \
+	        CFLAGS="-std=c11 -pthread -Wall -Werror -Iinclude -Isrc/word_source -Isrc/test_runner/include -DSTRICT_PTR=1 -march=rv64gc -mabi=lp64d -mcmodel=medany -DARCH_RISCV64=1 -O3 -DUSE_ASM_OPT=1 -DUSE_DIRECT_THREADING=1 -DNDEBUG -flto -static" \
+	        LDFLAGS="-flto -s -static -fuse-ld=lld" \
+	        all
+	@echo "✓ Cross-compiled binary ready: $(call binary_path,fastest,riscv64)"
+	@echo "  Run: qemu-riscv64 $(call binary_path,fastest,riscv64)"
+
 # Minimal/embedded build
 minimal:
 	$(MAKE) MINIMAL=1
@@ -1267,6 +1284,7 @@ help:
 	@echo "  rpi4            - Native build on Raspberry Pi 4"
 	@echo "  rpi4-cross      - Cross-compile from x86_64"
 	@echo "  rpi4-fastest    - Maximum optimization for RPi4"
+	@echo "  riscv64-clang   - Cross-compile for RISC-V 64 via clang (GCC fails here)"
 	@echo "  minimal         - Minimal/embedded build"
 	@echo ""
 	@echo "🔧 DEBUG & DEVELOPMENT:"
@@ -1322,6 +1340,7 @@ help:
 	@echo "📋 EXAMPLES:"
 	@echo "  make fastest                    # Fastest build for current platform"
 	@echo "  make rpi4-cross                 # Cross-compile for Raspberry Pi 4"
+	@echo "  make riscv64-clang              # Cross-compile for RISC-V 64 (clang)"
 	@echo "  make asm                        # Generate assembly for inspection"
 	@echo "  make debug                      # Debug build"
 	@echo "  make PREFIX=/usr/local install  # Install system-wide"