Correct the collision table's extent: 17 entries, deltas rise then fall

+5932 is the table's start, not a point inside it: walking backwards, no
predecessor satisfies "ascending with a delta under 4096". The run is 17 entries,
not 19 -- the earlier 19 counted two entries past the jump.

The deltas are

  204 212 216 224 224 228 228 228 224 220 216 200 96 96 100 96

then a jump of 41484 to 190508. The deltas rise and then fall -- a symmetric
profile, not a monotone one. That weakens the prefix-sum reading: a cumulative
offset table has no reason to come back down, whereas cell contents that grow and
shrink as the table walks across a level would.

17 against a 75-cell grid, and 75 - 17 = 58. No arithmetic relating the two has
been found. The earlier guess that 19 related to 3x25 did not survive
re-measurement and is not carried forward.

Still open. 17 entries may not correspond to the grid at all -- the table may
cover something else, or a subset of cells. Resolving it needs either the same
offset found in other levels at a position the leading counts predict, or a
reason for 17. Neither exists yet, and this is where the measurement trail goes
cold on collision.

docs/plan/level-assets.md

@@ -1553,7 +1553,26 @@ grid cells, so it is not one offset per cell -- either most cells share an
 offset, are empty, or the table covers something other than the grid. Nothing
 measured here picks between those.
 
-**The next thing to check** is whether level 4's run length, 19, relates to its
+**Followed up.** `+5932` is the table's *start*, not a point inside it: walking
-`3 x 25` grid by any arithmetic, and whether the same offset at `+5932` exists
+backwards, no predecessor satisfies "ascending with a delta under 4,096". The
-in the other levels at a position given by the leading counts. Neither has been
+run is **17 entries**, not 19 -- the earlier 19 counted two entries past the
-tried yet.
+jump.
+
+The deltas are
+
+    204 212 216 224 224 228 228 228 224 220 216 200 96 96 100 96
+
+then a jump of 41,484 to 190,508. So the deltas **rise and then fall** -- a
+symmetric profile, not a monotone one. That weakens the prefix-sum reading
+somewhat: a cumulative offset table has no reason to come back down, whereas
+cell contents that grow and shrink as the table walks across a level would.
+
+**17 against a 75-cell grid, and 75 - 17 = 58.** No arithmetic relating the two
+has been found. The earlier guess that 19 related to `3 x 25` did not survive
+re-measurement and should not be carried forward.
+
+**Still open.** 17 entries may not correspond to the grid at all -- the table may
+cover something else, or cover a subset of cells. Resolving it needs either the
+same offset found in other levels at a position the leading counts predict, or a
+reason for 17. Neither exists yet, and this is the point at which the
+measurement trail goes cold on collision.