Shape a text once per width, not once per ask

A container measures a child by drawing it in a box it may not keep, so
one layout asks a text for a dozen widths and comes back to widths it
has already had -- the hottest text in the depth-8 tree draws 32 times.
Each ask re-ran the shaper, because the two caches in front of it held
one entry each and a trial width alternating with a final width evicts
the answer about to be wanted again. `perf record` put 63% of a resize
frame in text and 0.9% in `draw_inner`.

So keep more than one: a bounded store of shapings on `TextData`, keyed
by the text, the attrs and the width, holding the parley layout and the
glyphs placed from it. Bounding the store rather than each buffer is
what keeps it a fixed cost -- +4 MB on a tree of 4,000 texts, which is
19 MB less than the code before #16 holds after the same resizes.

`TextBuffer` now holds the glyphs of the shaping it is drawn as, which
is where `TextView::tex` was. That leaves one place to invalidate rather
than two, so the `MutDetect` flags on a view's text and attrs have no
reader and go, along with the `buf.changed = true` after every edit.

On a 40-row tree of distinct random paragraphs, 500 resize frames:
124.2M instructions per frame before, 17.7M after, and 45.9M when the
width never repeats. The five reference renders and the resize render
are byte-identical, and the 100-seed sweep passes.

`tests/revision_cost.rs` is that tree, written in the API subset
`43ce8c7` shares so the same source measures the code this replaced.
Report the worst frame and p99 beside the median, since a stutter is
what somebody sees. Count glyph placements, and count a text render per
ask rather than per shaping, so the store cannot hide how many times a
layout drew the same text.

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

core/src/layout_diagnostics.rs

@@ -54,10 +54,11 @@ pub(crate) enum Counter {
     TextRenders,
     TextShapeHits,
     TextShapes,
+    GlyphPlacements,
 }
 
 impl Counter {
-    const COUNT: usize = Self::TextShapes as usize + 1;
+    const COUNT: usize = Self::GlyphPlacements as usize + 1;
 
     const NAMES: [&'static str; Self::COUNT] = [
         "updates",
@@ -89,6 +90,7 @@ impl Counter {
         "text renders",
         "text shape hits",
         "text shapes",
+        "glyph placements",
     ];
 }