Add new regression and ensemble experiment configurations for V2-M and OrthoBridge

- Introduced multiple regression experiment configurations targeting vf_md, including:
  - cd_solo_reg_set.json: CD tower only regression setup.
  - img_solo_reg_set.json: Image tower only regression setup.
  - reg_head_epoch_sweep.json: Baseline regression sweeps at different epochs (50, 75, 100).
  - reg_head_set.json: Various regression setups including baseline and OrthoBridge configurations.
  - single_eye_reg.json: Single-eye regression setup for worst-eye aggregation analysis.

- Added ensemble configurations for OrthoBridge with different inner bridges:
  - ortho_alts_ensemble.json: Ensemble tests with ConcatBridge, PairwiseAdditiveBridge, and GatedAdditiveBridge.
  - ortho_alts_tritower.json: Tritower tests with the same inner bridges.

- Created V2-M specific configurations:
  - baseline_reg_nt50.json: Regression baseline with V2-M backbone.
  - geom_vec_gt.json and geom_vec_unet.json: Geometry vector injection experiments with V2-M.
  - single_l1_bridges.json: Single-eye ensemble experiments with various bridge types.
  - tritower_geom_gt.json: Tritower setup with GT contour-rasterized masks.

- Promoted existing experiments to higher repetitions for robustness.

v4/scripts/experiments/phase3_v4/single_eye_grid.json

@@ -0,0 +1,39 @@
+[
+  {
+    "_note": "Single-eye + all_losses + Hadamard FusionBridge. Pairs with the in-flight ensemble_single_refugelike (single+BCD+Hadamard) to isolate the BCD-vs-all-losses effect. v3 phase 3 originally showed BCD generalizes better; this re-establishes it in v4.",
+    "run_name": "experiments/phase3_v4/single_all_losses_hadamard",
+    "reps": 10,
+    "overrides": {
+      "training": { "tower_loss_mode": "all_losses" }
+    }
+  },
+
+  {
+    "_note": "Single-eye + BCD + ConcatBridge. Pairs with in-flight baseline (single+BCD+Hadamard) to isolate the bridge effect. Justifies why we use FusionBridge (Hadamard) as default.",
+    "run_name": "experiments/phase3_v4/single_bcd_concat",
+    "reps": 10,
+    "stage_overrides": {
+      "nt": {
+        "module": "v4.classes.bridges.concat_bridge",
+        "class":  "ConcatBridge",
+        "args":   { "fusion_dim": 256 }
+      }
+    }
+  },
+
+  {
+    "_note": "Single-eye + all_losses + ConcatBridge. Fourth cell of the BCD-vs-all-losses × Hadamard-vs-Concat 2x2.",
+    "run_name": "experiments/phase3_v4/single_all_losses_concat",
+    "reps": 10,
+    "overrides": {
+      "training": { "tower_loss_mode": "all_losses" }
+    },
+    "stage_overrides": {
+      "nt": {
+        "module": "v4.classes.bridges.concat_bridge",
+        "class":  "ConcatBridge",
+        "args":   { "fusion_dim": 256 }
+      }
+    }
+  }
+]