docs+test(web): name and pin the @attribute retro-paint (review round 3)

Review found that adding ("attribute", fg(3)) for HTML/CSS also colours the
@attribute capture three already-bundled grammars emit — rust (attribute_item),
lua (<const>), yaml (directives) — which were previously unpainted. Verified on
a Rust buffer: #[derive(Debug)] now paints uniformly yellow (fg 3), an
improvement over unpainted and the distinct-attribute convention most editors
follow.

Name this retro-paint as intended in the framing (Q#WEB4, rev 4) and pin it with
rust_attribute_repaints_via_shared_attribute_capture so it is a chosen effect,
not incidental. @tag is unaffected (HTML/CSS only).

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

docs/web-grammars-html-css-framing.md

@@ -1,11 +1,11 @@
 # Web grammars (HTML + CSS) + HTML injections — framing
 
-**Revision 3 — pre-implementation. Ground truth: canonical `main` @
+**Revision 4 — implemented on branch `web-grammars` (PR #146). Ground truth:
-`4daa1b8` (after LaTeX #144 and inline-math-docs #145), 2026-07-23. Status:
+canonical `main` @ `4daa1b8` (after LaTeX #144 and inline-math-docs #145),
-framing only; no implementation.** Rev 2 settled the capture set, injection
+2026-07-23.** Rev 2 settled the capture set, injection scope, and LSP claim
-scope, and LSP claim (round 1); rev 3 corrects the `#match?` predicate analysis
+(round 1); rev 3 corrected the `#match?` predicate analysis (round 2); rev 4
-(tree-sitter evaluates it natively) and refreshes the folding footprint to the
+names the intended `@attribute` retro-paint on the already-bundled rust/lua/yaml
-current branch (round 2). See §0.1.
+grammars — verified on a Rust buffer and pinned by a test (round 3). See §0.1.
 
 ## 0.1 Revision history
 
@@ -44,6 +44,17 @@ current branch (round 2). See §0.1.
   `overlay.rs`/`daemon.rs`/`syntax.rs`. §0 now lists the real set; the
   zero-overlap conclusion is unchanged (stronger, if anything).
 
+### Round 3 (rev 3 → rev 4)
+
+- **R3-1 (medium).** Q#WEB4 verified which captures pmacs *lacks* but not the
+  reverse: which already-bundled grammars *use* `@attribute`. Three do — rust
+  (`attribute_item`), lua (`<const>`), yaml (directives) — so the new
+  `("attribute", fg(3))` entry retro-paints their previously-unpainted spans
+  yellow. Named in Q#WEB4 as intended (verified on a Rust buffer: uniform
+  yellow, an improvement over unpainted), and pinned by
+  `rust_attribute_repaints_via_shared_attribute_capture`. `tag` is unaffected
+  (HTML/CSS only).
+
 Add tree-sitter **HTML** and **CSS** grammars so `.html`/`.htm` and `.css`
 buffers get lexical highlighting, and — the north-star payoff — light up
 HTML's embedded-language **injections**: `<script>` → JavaScript (already
@@ -197,6 +208,19 @@ There is **no** `tag.delimiter` (rev-1 speculation, removed); HTML's `<`/`>`/
 - **general** — `tag`/`attribute` are standard tree-sitter web captures, so it
   also serves future html-ish grammars (vue/svelte/astro).
 
+**Retro-paint on already-bundled languages (intended, added rev 4).** The
+capture table is global, so adding `attribute` also colours the `@attribute`
+capture that three bundled grammars already emit but which was previously
+unrecognized (and so unpainted): **rust** (`attribute_item`/`inner_attribute_item`
+— every `#[derive(…)]` / `#![…]`), **lua** (the `<const>`/`<close>` variable
+attribute), and **yaml** (`%YAML`/`%TAG` directives). On merge these begin
+painting `attribute` yellow (`fg 3`). Verified on a Rust buffer: `#[derive(Debug)]`
+paints uniformly yellow (the `attribute_item` span carries no narrower overriding
+captures) — a distinct-attribute convention most editors follow, and an
+improvement over unpainted. This is **chosen, not incidental**: it is pinned by
+`rust_attribute_repaints_via_shared_attribute_capture` (`src/highlight.rs`).
+`tag` is clean — only HTML/CSS use it, so it has no retro-effect.
+
 **On CSS custom properties (corrected in rev 3):** pmacs passes the buffer text
 to `QueryCursor::captures` (`src/syntax.rs:1701`), so tree-sitter evaluates the
 standard `#match?`/`#eq?`/`#any-of?` predicates natively — pmacs special-cases

src/highlight.rs

@@ -1416,6 +1416,61 @@ mod tests {
         );
     }
 
+    #[test]
+    fn rust_attribute_repaints_via_shared_attribute_capture() {
+        // Intended side effect, named in the framing (Q#WEB4): the
+        // `("attribute", fg(3))` entry added for HTML/CSS also colours the
+        // `@attribute` capture that tree-sitter-rust (attribute_item), -lua
+        // (`<const>`), and -yaml (directives) already emit. A Rust
+        // `#[derive(Debug)]` — previously unpainted, since `@attribute` was
+        // unrecognized — now paints the attribute style throughout. Pinned so
+        // the retro-paint on this repo's primary language is a chosen effect,
+        // not an incidental one.
+        use crate::buffer::{Buffer, BufferId, EditOp};
+        use crate::cell::{Cell, CellSize};
+        use crate::syntax::{ParseView, SyntaxRegistry};
+
+        let reg = SyntaxRegistry::new();
+        let language = reg.language("rust").expect("rust grammar");
+        let src = b"#[derive(Debug)]\nstruct S;\n";
+        let mut buf = Buffer::new(BufferId::next(), "a.rs");
+        buf.apply_edit(EditOp::Insert { pos: 0, bytes: src })
+            .unwrap();
+        let view = ParseView::new(&buf, language, "rust".to_owned());
+        let handle = view.handle();
+        let _vid = buf.attach_view(Box::new(view));
+        let mut req = handle.make_request();
+        req.injection_aliases = reg.injection_alias_snapshot();
+        let bundle = crate::syntax::run_parse(req).expect("rust parse");
+        handle.install(reg.resolve_layer_queries(&bundle));
+
+        let mut hv = SyntaxHighlightView::new(handle, reg.theme());
+        let (rows, cols) = (1usize, 20usize);
+        let mut backing: Vec<Cell> = vec![Cell::default(); rows * cols];
+        let mut grid = CellGrid {
+            cells: &mut backing,
+            stride: cols as u32,
+            size: CellSize::new(rows as u32, cols as u32),
+        };
+        let viewport = Viewport {
+            buffer_start: 0,
+            buffer_end: u64::MAX,
+            cell_origin: CellCoord::new(0, 0),
+            cell_size: CellSize::new(rows as u32, cols as u32),
+            gutter_w: 0,
+        };
+        let registry = buf;
+        hv.render(&registry, viewport, &mut grid);
+
+        // `derive` (col 2) sits inside the `attribute_item` and paints the
+        // shared @attribute style (fg 3) — the intended retro-paint.
+        assert_eq!(
+            grid.get(CellCoord::new(0, 2)).style.fg,
+            pmacs_protocol::cell::Color::Indexed(3),
+            "a Rust #[derive] attribute paints the shared @attribute style (fg 3)"
+        );
+    }
+
     #[test]
     fn web_grid_paints_html_tag_and_attribute() {
         // Q#WEB4 acceptance: the two capture entries this lane adds (`tag`,