fix: skip packages without C benchmarks in benchmark-random-c

[Planeshifter]

• Read, understood, and followed the contributing guidelines

Summary

Fixes persistent random_benchmarks CI failures (~97% failure rate, 29/30 runs in the last 30 days).

Root cause

benchmark-random-c uses PACKAGES_PATTERN='manifest.json' to select random packages. WASM packages (e.g. @stdlib/blas/base/wasm/dasum, @stdlib/blas/base/wasm/dswap) carry a manifest.json listing their C dependencies but ship no benchmark/c/*.c file and require Emscripten to compile — which is not available in the CI runner. When one of these 47 WASM packages was selected from the pool of 1845, benchmark-c failed and the workflow aborted.

The very high failure rate (97%) reflects that at least one WASM package is statistically nearly certain to appear in 10 randomly drawn packages from 1845 when 47 are problematic.

Fix

Add a pre-flight find guard before calling benchmark-c. If a selected package has no *.c files under its benchmark/ directory, it is silently skipped.

 benchmark-random-c: $(NODE_MODULES)
 	$(QUIET) $(MAKE) -f $(this_file) -s list-random-lib-pkgs PACKAGES_PATTERN='manifest.json' | while read -r pkg; do \
+		if find "$$pkg/benchmark" -name "*.c" 2>/dev/null | grep -q .; then \
 			echo ""; \
 			echo "Running benchmark: $$pkg"; \
 			NODE_ENV="$(NODE_ENV_BENCHMARK)" \
 			NODE_PATH="$(NODE_PATH_BENCHMARK)" \
 			$(MAKE) -f $(this_file) benchmark-c BENCHMARKS_FILTER="$$pkg/.*" || exit 1; \
+		fi; \
 	done

This keeps the original PACKAGES_PATTERN='manifest.json' selection pool (changing it to benchmark*.c would be excluded by FIND_LIB_PACKAGES_EXCLUDE_FLAGS which strips **/benchmark/* paths, silently returning 0 packages and running nothing).

Validation

• Guard tested: WASM packages have no *.c in benchmark/ (confirmed), math/blas packages do
• 3-agent review caught that FIND_LIB_PACKAGES_EXCLUDE_FLAGS excludes benchmark/ subdirs, ruling out the pattern-change approach

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