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/v2m_variants/baseline_reg_nt50.json

@@ -0,0 +1,36 @@
+[
+  {
+    "_note": "F6 V2-M: regression baseline_reg_nt50 at refuge V2-M. Same architecture as reg_head/baseline_reg_nt50 but with the V2-M backbone in place of refugelike.",
+    "run_name": "experiments/v2m_variants/baseline_reg_nt50_v2m",
+    "reps": 10,
+    "overrides": {
+      "label_filter": [0, 1, 2]
+    },
+    "tower_overrides": {
+      "img": { "args": { "backbone": "refuge_efficientnet_v2_m", "freeze_ratio": 0.0 } }
+    },
+    "stage_overrides": {
+      "nt": { "epochs": 50 },
+      "img_aux": {
+        "module": "v4.classes.heads.regression",
+        "class":  "RegressionHead",
+        "args":   { "dropout": 0.3, "target_key": "vf_md", "loss": "mse" }
+      },
+      "cd_aux": {
+        "module": "v4.classes.heads.regression",
+        "class":  "RegressionHead",
+        "args":   { "dropout": 0.3, "target_key": "vf_md", "loss": "mse" }
+      },
+      "nt_head": {
+        "module": "v4.classes.heads.regression",
+        "class":  "RegressionHead",
+        "args":   { "dropout": 0.3, "target_key": "vf_md", "loss": "mse" }
+      },
+      "hb_head": {
+        "module": "v4.classes.heads.regression",
+        "class":  "RegressionHead",
+        "args":   { "dropout": 0.3, "target_key": "vf_md", "loss": "mse" }
+      }
+    }
+  }
+]