hard benchmark: the 7B "fusion wins / no headroom" results were saturation artefacts

The easy task families saturated 7B (strings & arith at 1.00), so the earlier
7B nulls — moe: fusion 0.87 > union 0.84; directed ~= soup — could not separate
"refinements don't help at scale" from "tasks too easy at 7B". Adds a hard task
variant (hard: true in tasks.py: multi-step lists, Caesar ciphers / letter
transforms, multi-step & larger arithmetic; same family labels and answer
formats, threaded through make_tasks/train_specialist/runners; hard specialists
cache separately as spec_*_hard) and re-runs both experiments at 7B on Imperial
CX3 (one L40S, 24 min, unsaturated: arith ~0.48, strings 0.67, lists 0.34).

Both nulls flip back to the 0.5B ordering:
- Union beats fusion again: routing 0.500 > fusion 0.40 (soup 0.392 / ties
  0.400), the same 10-pt margin as 0.5B. Fusion dilutes the fragile strings
  specialist so hard (0.665 -> soup 0.300) that soup even trails the best single
  specialist (0.425); routing keeps it intact (0.670).
- Directed selection beats soup again: 0.492 > 0.392 (+10 pts), recovering most
  of routing's benefit from one deployable merged model (lifts strings to 0.630).

Correction to the earlier interpretation: the llm_moe_hpc "regime flip" and the
llm_directed_hpc "no headroom" null were driven by TASK SATURATION, not base
capability. The operative variable is headroom — "merge, don't average" (union >
fusion) and "directed sex" (selection > single blend) hold whenever there is room
to lose to dilution: a weak base (0.5B) OR hard tasks at a strong base (7B-hard).
Fusion only wins in the degenerate corner where easy tasks let a strong base
compose to the 1.00 ceiling. Vindicates E8's max > mean in real 7B weights once
saturation is controlled.

Default (easy) task behaviour is unchanged (hard defaults False). +1 hard-task
test (131 green). Excludes the 0.5B smoke bundle (a pipeline gate, not a
deliverable). Results in results/llm_{moe,directed}_hard_hpc/ (parquet gitignored).

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

configs/llm/moe_hard_hpc.yaml

@@ -0,0 +1,24 @@
+experiment: llm_moe_hard_hpc
+kind: llm_moe
+seed: 1
+n_replicates: 1
+
+# The HARD-benchmark firm-up (L40S). The easy families saturated 7B at 1.00 (strings/arith), so the
+# 7B moe/directed nulls could not distinguish "refinements don't help at scale" from "tasks too easy".
+# This run uses the HARDER task variant (hard: true) — longer lists + multi-step ops, Caesar ciphers /
+# letter transforms, multi-step & larger arithmetic — so 7B is NOT saturated and routing vs fusion has
+# real headroom to separate. Trains fresh HARD specialists (cached as spec_*_hard). Same operators as
+# moe_hpc; the question: does the 0.5B ordering (union > fusion) reappear at 7B once fusion can no
+# longer trivially compose to the ceiling?
+
+base_model: Qwen/Qwen2.5-7B-Instruct
+hard: true
+families: [lists, strings, arith]
+n_train: 800
+n_test: 200
+n_route: 48
+epochs: 3
+lora: {r: 16, alpha: 32}
+operators: [soup, ties, moe_oracle, moe_learned, max_merge]
+
+output: {dir: results/llm_moe_hard_hpc}