fix(td3): allocate K-wide gsp_label buffer for TD3 dtraj E2E path

[jdbloom]

Bug (reproduced 2026-07-08, job 2478)

A TD3 + GSP_PREDICTION_TARGET=delta_theta_traj + GSP_PREDICTION_HORIZON=5 + GSP_E2E_ENABLED=true cell crashed at ep0:

File ".../GSP-RL/gsp_rl/src/buffers/replay.py", line 141, in store_transition
    self.gsp_label_memory[mem_index] = ( ... )
ValueError: could not broadcast input array from shape (5,) into shape (1,)

Root cause

The K-step trajectory label is shape (K,) = (5,), but the TD3 path's ReplayBuffer was constructed without gsp_label_size, so it defaulted to the scalar width (1,). The DDQN/DQN branches already derived the label width from the K-wide head output (gsp_network_output); the TD3 branch (added in #27/#28) never got the same wiring.

Fix

One-line-of-logic change in the TD3 branch of Actor.build_networks (gsp_rl/src/actors/actor.py), mirroring the DDQN branch verbatim:

gsp_label_sz = int(self.gsp_network_output) if self.gsp_e2e_enabled else 1
self.networks['replay'] = ReplayBuffer(..., gsp_label_size=gsp_label_sz)

gsp_network_output is the config-resolved head width: GSP_PREDICTION_TARGET=delta_theta_traj + GSP_OUTPUT_KIND=delta_theta_traj → size == GSP_PREDICTION_HORIZON (K). No new mechanism — reuses the exact derivation the DDQN path already uses.

Unchanged paths

• DDQN/DQN branches: untouched (diff is confined to the TD3 branch).
• TD3 scalar / non-e2e: gsp_label_sz resolves to 1, which is the ReplayBuffer default — buffer allocation is byte-for-byte identical to prior runs.

Tests

Added two build_networks regression tests (config-driven, reproducing the real crash path):

• test_build_networks_TD3_e2e_dtraj_allocates_K_wide_label_buffer — K=5: asserts gsp_network_output==5, gsp_label_size==5, and that a shape-(5,) label round-trips through store_transition without the broadcast error. Fails on the pre-fix buffer (asserts 1 == 5).
• test_build_networks_TD3_e2e_scalar_label_buffer_unchanged — K=1: pins the width-1 legacy allocation and scalar round-trip.

Full GSP-RL suite: 570 passed, 4 deselected, coverage 80.9% (≥65% gate).

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