fix(dflash): roll back partial weight loads

[HUSRCF]

Fixes the partial DflashWeights::load leak reported in #640. The HTTP streaming and long-context throughput sections remain separate follow-ups.

Summary

• replace the fallible DflashLayerWeights { ... } literal with a per-field partial owner;
• roll back top-level tensors, completed layers, the in-progress layer, DFlash2 convolution weights, selector projection, and ensure_mq_signs failure;
• use WeightTensor::free_all so AWQ/Paro sidecars are included;
• reuse the same layer teardown helper for normal unload and rollback;
• add a thread-local deterministic load-failure hook and a dflash_smoke --fail-load-after N diagnostic.

The successful load order, tensor names, shapes, quant dispatch, and forward path are unchanged.

Validation

• cargo check --locked -p hipfire-runtime
• cargo test --locked -p hipfire-runtime --lib dflash — 19 passed
• cargo check --locked -p hipfire-runtime --example dflash_smoke --features deltanet
• gfx1100/ROCm 7.14 failure injection after 2, 10, 25, and 55 successful load steps:
  • every arm returned the injected failure;
  • retained HIP allocator heap stayed constant at 153,092,096 bytes rather than increasing with failure depth;
  • every arm immediately reloaded and fully unloaded the complete draft in the same process (LOAD_ROLLBACK_RETRY PASS).
• real serve path, Qwen3.8-27B target + MQ4 DFlash draft, greedy:
  • output: The capital of France is Paris.
  • τ=3.5; empty=0, runaway=0, attractor=0, runtime errors=0
  • prompt MD5 bbf2d0483ecacaa2eb7296f9c8083f95

Remaining affected-machine validation

The reporter's original 32 GB gfx1201 configuration should rerun MQ6-Pro at max_seq=262144. This PR remains Draft until that OOM → AR-fallback path confirms that the failed draft load no longer starves the fallback.

[Kaden-Schutt]

Superseded by #691, now on beta (aad88151d): same DflashWeights::load rollback plus the scratch ctors, the generic builder, emitter rollback, and one ctx-cap predicate. Hardware-verified in gate runs 33902688013 / 33905366422.