xtask: the wasm check compiles the web app; the web's SVG depth cap follows from its stack

`cargo xtask wasm` (part of `cargo xtask ci`) checked the wasm-safe crates but not
apps/vectorcraft-web, so web-only code was never compiled before a merge. It now runs
`cargo check --target wasm32-unknown-unknown -p vectorcraft-web` too (no Trunk, no network).

The SVG importer's cap on the web was 48, sized from native measurements. rustc links the web
build with a 1 MiB stack (`-z stack-size=1048576`; the app passes no linker arguments of its
own) and usvg uses about 15 kB a level: half the stack at that cost gives 34, set in one place
with the arithmetic.

crates/svg/src/import/depth.rs

@@ -20,8 +20,22 @@ use super::css::{Styles, XNode, parse_decls};
 /// that hold one object aren't kept, and what is still deeper is merged into the group around
 /// it). usvg uses about 15 kB of stack for each level: the importer runs on a thread with 16 MB
 /// ([`vectorcraft_doc::depth::with_deep_stack`]), except on the web, where there is the one
-/// 1 MB stack.
-pub const MAX_ELEMENT_DEPTH: usize = if cfg!(target_arch = "wasm32") { 48 } else { 256 };
+/// stack: [`WEB_DEPTH`] there.
+pub const MAX_ELEMENT_DEPTH: usize = if cfg!(target_arch = "wasm32") { WEB_DEPTH } else { 256 };
+
+/// The stack of the web build: rustc links `wasm32-unknown-unknown` with
+/// `-z stack-size=1048576` (`rustc --print link-args` shows it), and the web app gives the linker
+/// no arguments of its own (`apps/vectorcraft-web`: `Trunk.toml`, `index.html`, no rustflags).
+const WEB_STACK: usize = 1 << 20;
+
+/// The stack usvg uses for each level of elements: about 70 levels overflowed the 1 MB main
+/// thread of Windows. Measured natively, never in a browser.
+const LEVEL_STACK: usize = 15 << 10;
+
+/// The deepest elements may nest on the web: half its stack at usvg's cost a level, the other
+/// half left for what is on the stack when a file is opened and for the cost being a native
+/// measure. 1,048,576 / (2 × 15,360) = 34.
+const WEB_DEPTH: usize = WEB_STACK / (2 * LEVEL_STACK);
 
 /// Properties whose value an element's descendants use too: the paint server they name is
 /// converted at each descendant, under everything between.

xtask/src/main.rs

@@ -25,7 +25,7 @@ commands:
   assets          check that every icon/image/font/asset is attributed in ASSETS.md
   brands          check that user-visible text (labels, params docs, MCP, docs, packaging) names no other vendor's products
   layers          enforce the crate dependency layering (plan/architecture.md §3)
-  wasm            cargo check --target wasm32-unknown-unknown for the wasm-safe crates
+  wasm            cargo check --target wasm32-unknown-unknown for the wasm-safe crates and the web app
   ci              fmt --check, clippy -D warnings, test, assets, brands, layers, wasm (stops at first failure)
   corpus [--download] [--affinity]
                   show where test corpora live; --download fetches PngSuite into corpus/pngsuite,
@@ -141,8 +141,8 @@ fn cmd_layers() -> Result<(), String> {
     }
 }
 
-/// Workspace packages that must build for wasm32: all L0–L5 crates plus
-/// the egui shell.
+/// Workspace packages that must build for wasm32: all L0–L5 crates, the egui
+/// shell, and the web app (its wasm-only code is compiled by nothing else).
 fn wasm_set() -> Result<Vec<String>, String> {
     let crates = layers::from_metadata(&metadata()?)?;
     Ok(crates
@@ -150,7 +150,7 @@ fn wasm_set() -> Result<Vec<String>, String> {
         .filter(|c| match layers::classify(&c.name) {
             Some(layers::Class::Layer(l)) => l <= 5 || layers::short_name(&c.name) == "ui-egui",
             Some(layers::Class::Standalone) => true,
-            _ => false,
+            _ => layers::short_name(&c.name) == "web",
         })
         .map(|c| c.name)
         .collect())