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_bridge_variants.json

@@ -0,0 +1,27 @@
+[
+  {
+    "_note": "Single-eye img+cd ensemble at refugelike with PairwiseAdditiveBridge at nt (eye-level fusion). For F3 confidence-strip panel expansion.",
+    "run_name": "experiments/phase3_v4/single_bcd_pairwise",
+    "reps": 10,
+    "stage_overrides": {
+      "nt": {
+        "module": "v4.classes.bridges.pairwise_bridge",
+        "class":  "PairwiseAdditiveBridge",
+        "args":   { "fusion_dim": 256 }
+      }
+    }
+  },
+
+  {
+    "_note": "Single-eye img+cd ensemble at refugelike with GatedAdditiveBridge at nt.",
+    "run_name": "experiments/phase3_v4/single_bcd_gated",
+    "reps": 10,
+    "stage_overrides": {
+      "nt": {
+        "module": "v4.classes.bridges.gated_bridge",
+        "class":  "GatedAdditiveBridge",
+        "args":   { "fusion_dim": 256 }
+      }
+    }
+  }
+]