clarify: session reference LABELS an all-contact episode's own clean frames (ep003 left = 26, not 0)

data/motherboard/reference/validation.md

@@ -2,7 +2,7 @@
 
 **Design (corrected after a drift check):** PRIMARY reference = per-episode fuzzy-mode (`fuzzy_mode_background`), drift-free and robust to partial contact. FALLBACK = per-session mode (`session_reference`, `<date>/tactile_*.png`) only for episodes that are ~100% contact (own mode contaminated).
 
-A single session reference is NOT a wholesale replacement: intra-day gel drift makes it differ from a given episode by a diffuse ~0.37 area with no object (see `drift_check.png`), so per-episode mode is preferred whenever the episode has clean frames.
+**Every episode-side has a non-empty pool (see below).** A single session reference is NOT a wholesale replacement: intra-day gel drift makes it differ from a given episode by a diffuse ~0.37 area with no object (see `drift_check.png`), so per-episode mode is preferred whenever the episode has clean frames.
 
 **Over 64 episode-sides:** need session fallback (no clean frame of own): **1** ['003 left']. Old p01 anchors that were a contact/marginal frame the per-episode MODE fixes: **0** [].
 
@@ -71,4 +71,21 @@ A single session reference is NOT a wholesale replacement: intra-day gel drift m
 | 002 | 2026-05-10 | right | 10475 | 7455 | 71.2 | 0.0 |  |
 | 003 | 2026-05-11 | right | 10032 | 7148 | 71.3 | 0.0 |  |
 | 013 | 2026-05-11 | left | 12724 | 9224 | 72.5 | 0.001 |  |
-| 001 | 2026-05-10 | right | 7621 | 5799 | 76.1 | 0.0 |  |
+| 001 | 2026-05-10 | right | 7621 | 5799 | 76.1 | 0.0 |  |
+
+## No episode has an empty pool
+
+The per-episode mode gives a **healthy pool for 63/64 sides** (median ~2145
+no-contact frames, ~56% of the clip). The one exception, `ep003 left` (121
+frames, gel in contact its entire length), yields 0 frames vs its OWN mode
+(its mode is contaminated by the held object) — but this does NOT mean it has no
+clean frames. Two-pass labeling recovers them:
+
+  1. label the pool with the per-episode `fuzzy_mode_background` (drift-free);
+  2. if that pool is ~empty, RE-label with `session_reference` — which finds the
+     episode's **own** clean frames.
+
+For `ep003 left`, the session reference recovers **26 real no-contact frames**
+(21%; indices 17–21 and 53–73) from within the episode. So the session reference
+is used to *label* an all-contact episode's own clean frames, not to substitute a
+synthetic reference. Result: **every episode-side has a real no-contact pool.**