Measure what truncating costs instead of asserting it

This section said truncating "breaks the one rounding the grid is for" and
left it at that, which is the same fault it withdraws the fused multiply-add
for: a claim nobody ran.

Run, it fails one exactness test by a step -- a share reaching 199.99902 of a
200px row -- and does not fail the warm-against-cold oracle any harder than
the head, which gives the same three failures on the same seeds within
0.002px. So the 57M cycles are not obviously bought with layout drift, and
whether a share stopping a thousandth of a pixel short is worth them is a
decision rather than a defect.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/HANDOFF.md

@@ -269,10 +269,22 @@ fifths of the instruction gap and two thirds to three quarters of the cycle
 gap, which is the looser number. The mechanism is visible in `nm`:
 `<UiSpan>::within` is a symbol in `4febabf` and in neither `5ed9e87` nor the
 all-three build. Shortening its body past the inliner's threshold is what
-pays, and only the whole set of cuts reaches it. None of them is shippable
--- truncating breaks the one rounding the grid is for -- but that is where
-to push, and a cheaper `Holds` or a smaller `Len::within` is worth trying
-for this reason rather than for its own instruction count.
+pays, and only the whole set of cuts reaches it. That is where to push: a
+cheaper `Holds` or a smaller `Len::within` is worth trying for this reason
+rather than for its own instruction count.
+
+**What truncating costs was then measured rather than asserted.** The first
+write-up of this section said it "breaks the one rounding the grid is for"
+and left it there, which is the fault it withdraws the fused multiply-add
+for. Run, it fails one exactness test by a step --
+`an_uneven_nesting_still_gives_every_share_the_same_length`, where a share
+reaches 199.99902 of a 200px row -- and it does **not** fail the
+warm-against-cold oracle any harder than the head does: 100 seeds pass, and
+1000 seeds of depth 6 give the same three failures on the same seeds,
+differing by 0.002px. So the case against it is a share stopping a
+thousandth of a pixel short of its row, which is below what an even number
+of pixels draws anyway, rather than layout drift. Whether that buys 57M
+cycles is a decision, not a defect.
 
 **Cycle noise is larger than this section used to claim.** Across ten sets
 of nine to eleven runs, each of one unchanged binary, cycles spread 1-3%