Record the snap as approved and CPU rounding as recommended

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/HANDOFF.md

@@ -218,8 +218,10 @@ fn snap_floor(v: vec2<f32>) -> vec2<f32> {
 }
 ```
 
-A rendering decision, so proposed rather than made. The check is the
+Approved by Bryan on 2026-09-17, together with rounding on the CPU; see
-reference render set plus the oracle; expect `tabs` to move its arcs back.
+item 5 under "Next" for why both, and what each does not fix. The check is
+the reference render set plus the oracle; expect `tabs` to move its arcs
+back.
 
 ### Smaller items
 
@@ -641,8 +643,21 @@ In order, from the review above and Bryan's steer (2026-09-17):
 3. Keep the `DrawInfo` on `ActiveData`; delete the copied fields and the
    reconstruction in `redraw`.
 4. `Span`'s leftover boundary through `Holds::through`.
-5. The shader snap, if Bryan takes it: change, then the render set and the
+5. **Round on the CPU and snap to the nearest pixel in the shader**, as
-   oracle.
+   one change with one verification. Bryan approved the snap on 2026-09-17
+   (rendering may change wherever it brings the screen closer to what the
+   user's code says: three equal sections of 1000 px need one of them
+   rounded up). CPU rounding is recommended for a different reason: a
+   `Rel` is off by at most `2^-25` of its box, so with round-to-nearest
+   every product whose true value is a whole number of steps is exact for
+   boxes under about 8,000 px, where truncation leaves half of them one
+   step short and layout then decides "does not fit" on a container the
+   user meant to fit exactly. Use the branchless round-half-up form,
+   `(a * b + (1 << (BY - 1))) >> BY`, not the sign-branching
+   `shift_round`; re-derive `Holds::through` for `round` (its two shifted
+   bounds move by half a `Rel` step); check with `nm` that `UiSpan::within`
+   still inlines; expect a couple of percent of instructions and re-run the
+   long fuzzers and the render set once for both.
 6. The smaller items: the stale `f32` comment, the gap of an undrawn child,
    confirm nested `leftover` weights, one zero-divisor fallback.
 7. `LazySpan`, the next LAYOUT.md §2 item. Region nodes cover the movable