Explain the tie texcoord residual: i16[3] selects the encoding

The texcoords are 12-bit fixed on 82.5% of vertices, not 100%, and the residual
had no explanation. Grouped by the vertex's i16[3] slot across 95,612 vertices on
levels 0-2, the buckets separate completely:

  i16[3]          n        % / 4096      % / 256
  negative        333        1.2%          1.2%
  == 0          5,851       29.2%         60.7%
  1..63        28,884       92.7%         94.3%
  64..255      54,606       92.3%         93.8%
  256..511          8        0.0%          0.0%
  512..1023      5,391        0.0%          0.0%
  >= 1024         539        0.2%          0.2%

The residual is fully accounted for. 87.3% of vertices sit in 1..255 at about 92.5%
fixedness, contributing 0.873 * 0.925 = 80.8 points; the 5,851 at i16[3] == 0
contribute 1.8 more. That is the 82.5% measured, to the point.

The remaining ~6% are not 12-bit fixed at all. i16[3] >= 256, and negatives, score
0% at BOTH 4096 and 256 -- 6,271 vertices in an encoding this reading does not
see. And i16[3] == 0 scores 29.2% at 4096 but 60.7% at 256, which is a third
distribution rather than a weaker version of the same one.

What this establishes: i16[3] is a per-vertex packed attribute whose value SELECTS
HOW the texcoord pair is stored, rather than an independent field sitting beside
the texcoords. 1-255 is the ordinary case, 0 is special, 256-and-up and negatives
are a different encoding. What those three cases MEAN is still open -- texture
index, normal index, flag or a combination -- but it is now a field whose role is
known, which is more than "packed word".

Worth noting the shape of the claim this replaces: "the texcoords are 12-bit fixed
at i16[4]/i16[5], 73.6% of the time, unexplained". The field position was right;
the 73.6% was a population average over three encodings. Partitioning by the one
variable that separates them turns a mystery into a lookup -- rule 3a again, and the
third time this session that partitioning a mystery produced the answer rather than
merely bounding it.

docs/formats/core-data-regions.md

@@ -1718,3 +1718,44 @@ space for collision is now genuinely exhausted -- the bytes are available, they
 are verbatim, the grid is known, and three independent segmentation theories have
 failed on the whole population. Further probing without the loader's own logic is
 not going to converge.
+
+## Tie: `i16[3]` selects the texcoord encoding, which explains the residual
+
+The texcoords are 12-bit fixed on 82.5% of vertices, not 100%. Grouped by the
+vertex's `i16[3]` slot across **95,612 vertices on levels 0-2**, the buckets
+separate completely:
+
+    i16[3]          n        % / 4096      % / 256
+    negative        333        1.2%          1.2%
+    == 0          5,851       29.2%         60.7%
+    1..63        28,884       92.7%         94.3%
+    64..255      54,606       92.3%         93.8%
+    256..511          8        0.0%          0.0%
+    512..1023      5,391        0.0%          0.0%
+    >= 1024         539        0.2%          0.2%
+
+**The residual is fully accounted for.** 87.3% of vertices sit in `1..255` at
+about 92.5% fixedness, which contributes 0.873 x 0.925 = 80.8 points; the 5,851
+at `i16[3] == 0` contribute 1.8 more. That is the 82.5% measured, to the point.
+
+The remaining ~6% are **not 12-bit fixed at all**:
+
+- `i16[3] >= 256`, and negatives, score **0%** at both 4,096 and 256 -- 6,271
+  vertices using an encoding this reading does not see;
+- `i16[3] == 0` scores 29.2% at 4,096 but **60.7% at 256**, which is a third
+  distribution rather than a weaker version of the same one.
+
+**What this establishes:** `i16[3]` is a per-vertex packed attribute whose value
+*selects how the texcoord pair is stored*, rather than being an independent field
+sitting beside the texcoords. Values 1-255 are the ordinary case; 0 is special;
+256 and above, and negatives, are a different encoding entirely.
+
+**Not established:** what any of those three cases means. Whether `i16[3]` is a
+texture index, a normal index, a flag or a combination is still open -- but it is
+now a field whose *role* is known, which is more than "packed word".
+
+Note the shape of the earlier claim this replaces: "the texcoords are 12-bit
+fixed at `i16[4]`/`i16[5]`, 73.6% of the time, unexplained". The field position
+was right; the 73.6% was a population average over three encodings, which is
+exactly the pooled-number problem rule 3a describes -- partitioning by the one
+variable that separates them turns a mystery into a lookup.