Record the seventh sweep, over the rigs and the sixth sweep's own fix

Six findings in iris f8aa0c5, and the seven things that tripped a rule and
were left. Three of those are named as work waiting for the pre-review-gate
pass, since they pre-date #19 and are outside its diff.

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

docs/LAYOUT_LOG.md

@@ -6,6 +6,117 @@ nothing here is rediscovered. Each entry says who found it and when.
 it (settled design, the measurement method) belongs in `docs/LAYOUT.md`, and
 the current plan is in `docs/HANDOFF.md`.
 
+## Seventh sweep: the rigs, `Fixed`, and the sixth sweep's fix (2026-09-20)
+
+Over what no earlier round named -- `src/random.rs` and `tests/scenario/`,
+`core/src/fixed.rs`, `scripts/run-headless.sh` -- and once more over
+`b7b8d09`, which was itself unreviewed because it was the sixth sweep's own
+fix. Six findings, all in `f8aa0c5`. The cold dump over 400 depth-5 trees is
+byte-identical to `b7b8d09` across all 34,492 boxes, and all three seed scans
+pass (400 at depth 5 in 62.75s, 1,000 at depth 6 in 162.37s, 2,000 at depth 4
+in 305.25s).
+
+**A scroll still asking a question it has already answered.** The sixth
+sweep found `anchor != Px::ZERO` in `Scroll`'s content test and removed it;
+the operand beside it is the same defect and survived. `self.content_len` is
+`answer_px.max(container_len)`, so content that fits has nothing to scroll
+through, and `update_amt` on the line above clamps `amt` to
+`content_len - container_len`, which is then zero. So in
+
+    let content = match self.amt != Px::ZERO || self.content_len != self.container_len {
+
+the first disjunct can never decide the match: `amt != ZERO` implies
+`content_len != container_len`, which the second already tests. It is now
+`match self.content_len > self.container_len`, the exact complement of the
+`content_len <= container_len` that guards the contract fifteen lines above.
+Verified by asserting the implication in place and running the whole suite,
+the scrolling tests included; it held. This is the lesson of "the fixes a
+review produces are themselves unreviewed code" arriving on schedule -- one
+round's fix left the same mistake in the expression it was editing.
+
+**Things nothing reads, in the new arithmetic.** `Fixed::to_scale` converts a
+value between two fixed-point grids, and `shift_round` exists only to serve
+it. Both arrived on this branch; the only caller either has ever had is
+`a_coarser_grid_rounds_and_a_finer_one_does_not`, the test written for them.
+Every other `Fixed` method has real callers (checked one by one), and nothing
+needs a grid conversion: `ratio`, `mul` and `div` already cross grids where
+layout has to. All three deleted. `to_scale` was also the one operation in
+the file that could overflow silently without documenting it -- going to a
+finer grid is `self.0 << (TO - SHIFT)` and the test only ever went coarse,
+fine, coarse with a value small enough to survive the trip.
+
+**`Len` arithmetic written a component at a time.** `Len::align` built a
+`Len` whose `px` is always `Px::ZERO`, then added to and subtracted from both
+of its components by hand:
+
+    start: Len::from_parts(at.rel.sub(self.rel.mul(rel)), at.px.sub(self.px.mul(rel))),
+    end:   Len::from_parts(at.rel.add(self.rel.mul(rest)), at.px.add(self.px.mul(rest))),
+
+`Len::scale` is "both parts by the same fraction" and `Len` has `Add` and
+`Sub`, so the whole rule is `at - self.scale(rel)` and
+`at + self.scale(Rel::ONE.sub(rel))` -- the point the alignment names, less
+the part of the length before it. Identical arithmetic, which the dump
+confirms. It is the only place in the codebase that expanded a `Len`
+operation like this; `LayoutLen::apply_leftover` touches `rel` alone, which
+is genuinely one component.
+
+**A question asked through a value, one line from its sibling.** `445287c`
+moved every method that answers a question about a value to `&self`.
+`LayoutLen::without_leftover` was missed, and its own doc comment calls
+`apply_leftover` -- which takes `&self` -- "the opposite reading of the same
+value". Now `&self` too. Every other by-value method in the workspace that
+does not return `Self` was checked: the rest are conversions on numbers
+(`Fixed::raw`, `to_f32`) or builders.
+
+**A rig that scales a gesture against a mode the output no longer has.**
+`run-headless.sh --mode` sets `out_w`/`out_h` beside the `swaymsg output ...
+mode`, because those two numbers are the extent `replay-touch` passes to
+`zwlr_virtual_pointer_v1::motion_absolute` -- the recording's coordinates are
+a fraction of them. `--resize`, added in `b7b8d09`'s round, changed the mode
+and left the extent alone, so `--resize 800x600@60Hz --replay flick.touch`
+replayed every sample at `x * 800 / 1920` and finished looking like a run
+that worked. Both are one `set_mode` function now, so a third mode change
+cannot get it wrong. Found by reading rather than by running: this VM has no
+recorded gesture that also resizes, which is exactly why it went unnoticed.
+
+**A comment the plan/build split stranded.** `src/random.rs`'s "a row takes
+the height it is given rather than its tallest child" sat above
+`let gap = self.rng.below(3) as i32 * 4`, telling a reader that the line
+consumes no randomness. It describes the `set_size_rules` that `98d4e98`
+moved into `Build::kind`, and the line it was left above is one of the two
+draws in the function. Moved to the rule it is about, and the
+seed-stability half dropped: building a plan consumes no randomness at all
+now, so there is nothing left for that clause to say.
+
+### Tripped a rule and left as it stands
+
+- `Align::tuple`, `UiVec2::partial_align` and `Vec2::partial_align` have no
+  callers anywhere. All three pre-date this PR and are outside its diff, so
+  they belong to the pre-review-gate sweep rather than to this branch.
+- `Vec2::align`/`partial_align` are `UiVec2`'s two functions again with
+  `UiVec2::from(*self)` in front. Also pre-existing, and merging them means
+  deciding whether `Vec2` should have them at all.
+- `impl_op!` has four grammars for one macro, and `core/src/util/vec2.rs`
+  spells two of them one line apart -- `impl_op!(impl Add for Vec2: add x y)`
+  beside `impl_op!(Vec2 Sub sub; x y)`. The `impl ... for ...:` arm has that
+  one caller. Pre-dates this PR; the two arms this PR added (`same ...`) are
+  a real distinction, since a type mixing a fraction and an offset has no
+  meaning for a bare `f32`.
+- `AxisAlign`, `RegionAlign` and `Align` lost `Eq` when `AxisAlign` became a
+  `Rel` wrapper, which `Rel` derives. Nothing needs it, so it was left rather
+  than adding a derive with no reader.
+- `Holds::through` special-cases a fully unbounded range before inverting a
+  fraction, which the general path would also answer correctly through
+  `narrow`. Left: it is the one place a `Px::MIN`-to-`Px::MAX` interval is
+  shifted and divided, and the early return says that no fraction can narrow
+  "every length".
+- `Scroll`'s `content_len <= container_len` can only be equality, given the
+  `max` that built it. Left: `<=` reads as "the content fits", which is what
+  the branch means, and the mirror `>` now reads as "it overflows".
+- `scenario::Case::name`, `window` and `Shuffle::of` take `self` on `Copy`
+  enums. They are test-rig code the `&self` pass did not cover, and an enum
+  with no fields is the one place taking a value costs a caller nothing.
+
 ## Sixth sweep: the shader boundary and the position widgets (2026-09-20)
 
 Over what the five earlier rounds did not name -- the WGSL prelude and how