Record the sixth sweep, and the base every sweep should have used

PR #19's base is upstream/main at ca2b4b2, not the local `main`, which
tracks the fork's divergent line and sits four merged pull requests behind
it. Diffing against `main` shows #10, #12, #16 and #17 as this branch's
work; that is how the parley text migration's undo path kept surfacing in
sweeps. Git cannot record a pull request's base and `main` cannot be
renamed, so the handoff carries the note and the checkout carries
`git iris-base`/`git iris-diff`.

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

docs/LAYOUT_LOG.md

@@ -6,6 +6,99 @@ 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`.
 
+## 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
+it is assembled, the position widgets, `orientation/`, and the sensor walk.
+**Scoped against `upstream/main` at `ca2b4b2`, which is PR #19's real base**;
+the first half of this sweep used the local `main` and had to be redone, for
+which see "The base is `upstream/main`" in `docs/HANDOFF.md`. Two findings.
+The cold dump is byte-identical to `1096c31` and all three seed scans pass
+(400 at depth 5 in 69.07s, 1,000 at depth 6 in 169.29s, 2,000 at depth 4 in
+300.75s).
+
+**A number both sides count in, written twice.** `module_source` already
+builds each shader's preamble from `iris_core`'s own constants, with a
+comment saying why: "a grid the two disagree about puts every coordinate
+somewhere else". The move-chain work then added, to the very file it
+prepends, a second copy of two of its own numbers -- `const MOVE_NONE` and
+`const CHAIN_LIMIT`, under "Keep in step with `iris_core::CHAIN_LIMIT`" --
+asking a reader by hand for what the mechanism beside it exists to do, and
+duplicating the reasoning `CHAIN_LIMIT`'s Rust declaration already carries.
+Both are injected now and the shader declares neither.
+`every_shader_validates` composes the real preamble so it covers the change;
+what it could never have caught is the two numbers drifting apart, which is
+now unrepresentable.
+
+`MASK_NONE` went in beside them, replacing a bare `4294967295u` in `masked`
+-- where the CPU deliberately keeps `MaskIdx` and `MoveIdx` as separate
+types so the two cannot be swapped. **That literal pre-dates this PR**; it
+is a one-line drive-by in a block the PR was already rewriting, taken
+because leaving it means a named sentinel for one index and a magic number
+for its sibling four lines apart. Drop it if the scope matters more.
+
+**A scroll positioning content it does not position.** Two halves, one
+mistake, both new in this PR -- the base's `Scroll` has none of this
+machinery. The author believed `Scroll` places its own content.
+
+`content_len` is `answer_px.max(container_len)`, so it is never less than
+the box. Two lines on, `slack` was `(container_len - content_len).max(ZERO)`
+and `anchor` was `slack * align.rel()` -- provably always zero, whatever the
+alignment, with `moved` then testing `anchor != ZERO` for nothing.
+Instrumented at `1096c31`, a centred scroll over 50 px of content in a
+200 px box prints `slack=0 align=AxisAlign(0.5) anchor=0` and centres the
+content anyway: the framework does it, by placing the inner's answer in the
+whole box, which comes out as `rel 0.5 - px 25` and resolves at any length.
+The comment credited the arithmetic for behaviour it could not produce.
+
+The same belief cost a redraw. The contract for content that fits was
+guarded by `align == AxisAlign::NEG`, on the reasoning that at any other
+alignment "it moves with every length the box takes and the drawing holds
+for that length alone". It does not move -- the framework's placement is a
+fraction of the box -- and the default alignment is the middle, so the
+common case was the guarded one. `Painter::px_len` holds a drawing to the
+length it read unless the widget says otherwise, so with no `holds` stated
+the scroll redrew on every box change. Measured with `distinct_widgets`: 1
+at `CENTER`, 0 at `TOP_LEFT`. Dropping the alignment test gives 0 at all
+three, which is what `a_fitting_scroll_holds_for_every_box_its_content_fits_in`
+asserts; it fails at `1096c31` on the `CENTER` case. `align` had no other
+reader, so the widget no longer asks its own alignment at all -- which is
+the tell that both halves were one mistake.
+
+**`UiSpan::translated` and `UiRegion::translated`** arrived on this branch
+and are reachable only from each other, which is why a plain "is this name
+used anywhere else" scan does not see them: neither looks unused on its own.
+The region one carried a performance argument for a function nobody calls.
+Both deleted.
+
+### Kept although they pre-date this PR
+
+`GlyphAtlas::glyph_count`, `TextBuffer::new_empty` and the let-chain in
+`TextEdit::apply_event` that #10 had expanded into a nested `if`. All three
+were found under the wrong base and are outside #19's diff; Bryan said to
+keep them anyway (2026-09-20), since the two are dead either way and the
+third is a straight restoration.
+
+### Tripped a rule and left as it stands
+
+- `diag::untrace_widget` is new here with no caller, which is the test the
+  `translated` pair was deleted by. Left: it is the "removed" half of what
+  `trace_widget`'s own doc promises ("until explicitly removed or cleared"),
+  and `clear_traced_widgets` is the "cleared" half and does have one. A rig
+  drives this API from outside.
+- `UiSpan::flip` swaps `start.rel` with `end.rel` and `start.px` with
+  `end.px`, where `Len` has exactly those two fields and one swap of the
+  structs would do. Pre-dates this PR.
+- `UiVec2` translates under three names -- `shift`, `offset`, and
+  `UiRegion::offset` -- beside `Len::offset`, which adds pixels to one end
+  and is a different operation. Pre-existing, and one vocabulary is Bryan's
+  call rather than a sweep's.
+- The nine `#[allow(unused_variables)]` are all on trait methods with empty
+  default bodies, where the parameter names are the signature's
+  documentation and underscoring them would hide it from implementors.
+- `GpuPages::update` grows before it drains and both go through the one
+  queue, so the copy is ordered before the writes. Correct as it stands.
+
 ## Fifth sweep: the retained path, the renderer and the diagnostics (2026-09-20)
 
 Over the parts the four earlier rounds did not read -- the renderer, the
@@ -79,8 +172,12 @@ throughout, `LayoutLen::{is_px, is_only_leftover, declared, fills}` and
   same words share an answer" means, which is a design question.
 - `TextEdit`'s undo history pushes a whole copy of the text per changed
   keystroke and is never bounded, and `apply_event` clones the text on every
-  event including the arrow keys. Both are on `main` verbatim and unchanged
-  by this branch, so they belong to a change of their own.
+  event including the arrow keys. **`apply_event` is not in this PR's diff at
+  all**: it was last touched by #10 and #16, both already on `upstream/main`.
+  It keeps resurfacing in sweeps because they diffed against the local `main`
+  -- see "The base is `upstream/main`" in `docs/HANDOFF.md`. Bryan wants the
+  unbounded push and the clone dealt with as a change of their own
+  (2026-09-20).
 - `ActivationState::update` writes four arms where the `Start`/`On` and
   `End`/`Off` pairs are identical, and `is_off` is `!is_on`. Also verbatim
   from `main`.