Say the environment once, and stop a hint read going uncounted

The eleventh sweep, over the built-in bounds work in 2ac0843.

`Painter::size_hint` refused to answer for a bounded widget by returning
above the diagnostics, so that read was neither a hint hit nor a miss and
`hint_read` recorded nothing. It is a miss now, with the reason on it.

The two `for axis in Axis::BOTH` loops that `draw_widget` grew, both
writing `own_holds`, are one loop, and the comment about combining the
ask's holds no longer sits between a comment and the code it describes.

`Declared::from_axes` lost its only caller with `Widgets::declared_lens`;
`Bounds::from_axes` and `SizeRule::declared` never had one.

The scenario shrinker printed a rule with derived `Debug`, which is 130
characters an axis in a line that carries every ancestor, in the one
function whose job is output a tree can be rebuilt from. It prints its
parts again.

`bounds_cost` invented three environment-reading spellings where four
copies of one `env` helper already existed; there is now one, in
`tests/rig`, and the four copies are gone. It also verified 128 regions
inside its measured loop, which the other rigs deliberately do before
theirs; that measured 0.65% of the total, and none of it is layout.

The 250-window row with a 300 cap was built by two tests, and the one
that still explained itself tested less; they are one. The half of
`a_cap_attribute_narrows_the_widgets_box` that the wrapper's removal left
without its deciding assertion is the allocator's path instead, which
nothing at the root covered.

Format, workspace clippy under -D warnings with and without
layout-diagnostics, 206 ordinary and 210 diagnostic tests, 400 depth-5
trees warm against cold in 64.19s, and the cold dump byte-identical to
2ac0843 across all 34,986 boxes.

tests/layout_diagnostics.rs

@@ -16,9 +16,12 @@
 //! `IRIS_DIRTY` how many widgets `many` marks at once. `IRIS_UNBOUNDED=1`
 //! removes intrinsic bounds while preserving the rest of the generated tree.
 
+mod rig;
+
 use iris::harness::Harness;
 use iris::prelude::*;
 use iris::random::{Edits, Tree, build, plan};
+use rig::env;
 use std::time::Instant;
 
 const OUTPUT: (f32, f32) = (1920.0, 1200.0);
@@ -94,13 +97,6 @@ fn a_selected_widget_retains_its_layout_events() {
     diagnostics::clear_traced_widgets();
 }
 
-fn env<T: std::str::FromStr>(name: &str, fallback: T) -> T {
-    std::env::var(name)
-        .ok()
-        .and_then(|value| value.parse().ok())
-        .unwrap_or(fallback)
-}
-
 #[cfg(feature = "layout-diagnostics")]
 fn trace_selected(tree: &Tree) {
     let Ok(value) = std::env::var("IRIS_TRACE_INDEX") else {