llm_merge_hpc: the 7B firm-up makes the Fisher-Muller sign decisive

Re-ran the specialist-merge experiment at a capable base (Qwen2.5-7B-Instruct,
200 tests/family) on one L40S GPU of Imperial's CX3 HPC (8 min walltime). The
two caveats the 0.5B prototype left marginal are now resolved:

- "exceeds every parent overall" is clean: both merges 0.87 vs best specialist
  0.77 (+10 pts), and above every specialist on every family.
- dilution vanishes: at 0.5B averaging diluted the lists-specialist
  (0.43->0.26); at 7B the merge beats it (0.62>0.57). Dilution was a
  small-model artefact -- a capable base composes rather than dilutes, which
  softens E4's "merge, don't average" once the parents are strong.

The figure title is now data-driven (reports ">" for 7B, "~" for 0.5B).
Adds the hpc/ smoke job script and the llm_merge walltime trim. Results synced
to results/llm_merge_hpc/ (parquet gitignored per the reproducibility contract).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

hpc/llm_smoke.pbs

@@ -0,0 +1,14 @@
+#!/bin/bash
+# Smoke gate: run the small 0.5B merge experiment on an L40S to confirm the GPU pipeline works
+# on CX3 before the 4 h 7B run. ~5 min.  submit: qsub hpc/llm_smoke.pbs
+#PBS -l select=1:ncpus=8:mem=48gb:ngpus=1:gpu_type=L40S
+#PBS -l walltime=00:20:00
+#PBS -N lam_llm_smoke
+
+cd "$PBS_O_WORKDIR"
+export HF_HOME="$EPHEMERAL/hf_cache"
+export TOKENIZERS_PARALLELISM=false
+source .venv/bin/activate
+nvidia-smi --query-gpu=name,memory.total,driver_version --format=csv,noheader
+python -m llm.experiment configs/llm/merge.yaml   # 0.5B, writes results/llm_merge/
+echo "done: $(date)"