Prune the docs of work already done: 18,252 -> 7,567 lines

Iris: "the documentation is also pretty crazy too. Can you go through it
and remove everything that's already done and decided? There's entire md
files iirc for projects already complete. And many with checkboxes already
ticked off that just fill up context."

  docs/RUST.md        8503 -> 905    the framework bake-off (options,
                                     recommendation, twelve closed
                                     experiment boxes) and two superseded
                                     "where things stand" sections, out;
                                     what the experiments settled kept as
                                     one line each
  docs/IRIS_TODO.md   1383 -> 229    fifty closed items and six
                                     phone-report sections whose defects
                                     are all fixed
  docs/LAYOUT.md      1116 -> 829    the pre-implementation framing: the
                                     old trait, the checklist, the
                                     migration list, the pass conditions
  docs/TEXTURES.md     496 -> 240    the prior-art survey, the proposal
                                     and its review, all implemented
  docs/REVIEW-*.md     673 -> 0      two completed review passes; the two
                                     findings left open on purpose (mask
                                     hit-testing, the phone's font set)
                                     moved into RUST.md

What survives a prune is what cannot be cheaply re-derived: measurements
(the APK-size table, the phone bench reports), dead ends, invariants and
their reasons, and the design of what exists now rather than the route to
it. AGENTS.md now says that, so the next session prunes as it goes rather
than appending; docs/IRIS_TODO.md's header says items are deleted when
they land rather than ticked.

Deleting the two review files left eighteen citations dangling in code
comments that state their reason inline and cited the file for provenance
only — those now read "(review, 2026-09-06)" and carry no dead pointer.
The emulator's measured GPU capabilities moved to the this-machine-android
skill, where machine facts belong. IRIS.md and DECISIONS.md are dated
records and were not rewritten; each gained one note that paths in older
entries predate the 2026-09-08 crate merge, pointing at the mapping.

Not touched, deliberately: docs/DECISIONS.md's entries (that file *is* the
queue of things for Iris to review, so deleting decided items would remove
what it exists for) and iris/readme.md and iris/TODO, which are hers.

Verified: ./run-tests.sh and `cd iris && cargo test` green, clippy and fmt
clean in every workspace, and every remaining docs/*.md cross-reference
resolves.

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

core/src/ui/painter.rs

@@ -131,7 +131,7 @@ impl<'a> Painter<'a> {
         // and the second call silently *replacing* the first is a widget
         // drawn unclipped -- which reaches the screen and nothing says so.
         // Every build anybody runs here is release
-        // (docs/REVIEW-2026-09-07.md's R1).
+        // (review, 2026-09-07).
         assert!(
             self.own_mask == MaskIdx::NONE || self.mask != self.own_mask,
             "set_mask called twice while drawing one widget: the second would replace the first \