Paper section: PD-sep scaffold + drop --enforce-eager from launch scripts

Adds analysis/pd_sep_paper_section/ as the home for the "PD separation is
net negative under agentic workloads" paper section: plot scripts for C1
(workload chars), C6 (roofline), C7 (routing-vs-PD-sep lever), the C6/C7
PDFs already rendered, and a README mapping candidate claims to required
figures plus open re-run items.

Removes --enforce-eager from bench.sh and all active launch scripts so
cuda graphs are captured -- the prior methodology suppressed one of
PD-sep's structural advantages (D-node fixed-shape decode). Legacy
scripts under scripts/legacy/ are intentionally untouched as historical
records.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

scripts/bench.sh

@@ -147,7 +147,7 @@ launch_instances() {
             $VLLM serve "$MODEL" \
                 --host 0.0.0.0 --port $port \
                 --tensor-parallel-size 1 \
-                --trust-remote-code --enable-prefix-caching --enforce-eager \
+                --trust-remote-code --enable-prefix-caching \
                 --dtype auto --gpu-memory-utilization 0.9 --max-model-len 200000 \
                 --kv-transfer-config '{"kv_connector":"MooncakeConnector","kv_role":"kv_both"}' \
                 $vllm_extra_args \
@@ -158,7 +158,7 @@ launch_instances() {
             $VLLM serve "$MODEL" \
                 --host 0.0.0.0 --port $port \
                 --tensor-parallel-size 1 \
-                --trust-remote-code --enable-prefix-caching --enforce-eager \
+                --trust-remote-code --enable-prefix-caching \
                 --dtype auto --gpu-memory-utilization 0.9 --max-model-len 200000 \
                 $vllm_extra_args \
                 > "$logfile" 2>&1 &