feat(paged): paged-bench - measure capacity & prefix-sharing wins

Quantify the two multi-tenant wins that are properties of the host-side
block model (vLLM-parity), independent of the in-model compute path:

  WIN 1 concurrency capacity @ 512-block budget
    contiguous (reserve n_ctx/seq): 4 sequences
    paged (on-demand blocks):       37 sequences
    --> 9.2x more concurrent sequences

  WIN 3 cross-tenant prefix sharing (32 tenants, 1024-tok shared prefix)
    prefix-cache OFF: 2176 physical blocks
    prefix-cache ON:  192 physical blocks
    --> 11.3x less KV memory

WIN 2 (throughput) is deliberately reported as PENDING: it requires the
paged gather-read path wired into llama-graph.cpp (Gate 0) and is not
measurable at the allocation layer. The win-1 baseline is per-sequence
n_ctx reservation (stream mode); llama.cpp's unified cache already shares
one pool, so the honest win there is on-demand sizing + prefix dedup.

Phase 3 (partial) of docs/superpowers/plans/2026-06-19-paged-attention-llamacpp.md.

Assisted-by: Claude:opus-4.8 [Claude Code]
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

backend/cpp/llama-cpp/paged/Makefile

@@ -12,6 +12,12 @@ tests/%: tests/%.cpp paged_kv_manager.cpp paged_kv_manager.h
 check: all
 	@for t in $(BINS); do echo "== $$t =="; ./$$t || exit 1; done
 
+paged-bench: paged-bench.cpp paged_kv_manager.cpp paged_kv_manager.h
+	$(CXX) $(CXXFLAGS) -o $@ paged-bench.cpp paged_kv_manager.cpp
+
+bench: paged-bench
+	./paged-bench
+
 # --- Optional ggml integration test (Phase 1: paged write/gather mechanism) ---
 # Requires a built ggml. Override these to point at your checkout / build:
 #   make ggml-check GGML_SRC=<llama.cpp>/ggml GGML_BUILD=<ggml-build>
@@ -30,6 +36,6 @@ ggml-check: $(GGML_BINS)
 	@for t in $(GGML_BINS); do echo "== $$t =="; ./$$t || exit 1; done
 
 clean:
-	rm -f $(BINS) $(GGML_BINS)
+	rm -f $(BINS) $(GGML_BINS) paged-bench
 
 .PHONY: all check ggml-check clean