Theme compile failures are structured hard errors, not silent DEFAULT_CSS (bd-jsvetdea)

[cscheid]

Summary

A grass failure while compiling the theme bundle used to be caught in variant_css, logged as a Warn trace event (which no CLI observer receives), and replaced by the static 7 KB DEFAULT_CSS. The render reported success. A one-line unit mistake in a user theme ($grid-body-width: 52rem) silently shipped an unstyled page.

Every theme compile failure is now a PipelineError::Structured through theme_diagnostic:

• Q-14-6 "Theme SCSS compilation failed": the grass / dart-sass text verbatim, anchored at the whole theme: value (the compiler only knows lines of the assembled bundle), with a hint saying so. Applies to the themed path, the no-user-input default bundle, and the reveal path (which used to fall back to the vendored stock theme).
• Q-14-7 "Theme file has no layer boundary markers": InvalidScssFile, anchored at the offending theme: entry when its resolved path matches one, else at the whole value.

Plumbing: the three compile wrappers keep SassError instead of stringifying it; InvalidScssFile gains a location field; theme_diagnostic gains sass_error_to_parse_error_at(err, fallback_location, candidates). Because the anchor is a _quarto.yml span, the render summary's source-location coalescing prints one report per project with an "Affected files" tail.

Closes bd-jsvetdea and bd-qmpygp02; resolves bd-36vmz7nk (decision: fatal). Follow-ups filed: bd-c1gf9xmk (layer provenance so Q-14-6 can name theme.scss:2), bd-e1psgogt (audit remaining Warn trace sites; -v filter never matches quarto_core).

Plan: claude-notes/plans/2026-09-08-theme-scss-compile-error-swallowed.md.

Verification

• Unit, diagnostic, and CLI e2e tests written first and observed failing on the old tree (exit 0 / Ok(DEFAULT_CSS) / no code).
• cargo nextest run --workspace: 13753 passed. Clippy and cargo xtask lint clean. hub-client npm run build:all (WASM leg) succeeds.
• End to end on the repro fixture (claude-notes/plans/theme-scss-compile-error-swallowed-investigation/repro/):

$ cargo run --bin q2 -- render <repro>
error: while rendering …/repro/index.qmd
Error: [Q-14-6] Theme SCSS compilation failed
   ╭─[ …/repro/_quarto.yml:5:12 ]
 5 │     theme: [cosmo, theme.scss]
   │            ─────────┬─────────
   │                     ╰─────────── compiling the theme SCSS bundle failed:
Error: Incompatible units px and rem.
3269 │ $grid-body-column-min: quarto-math.min(500px, $grid-body-column-max) !default;
./stdin:3269:24
ℹ Does a variable in a theme file set a value Bootstrap's arithmetic cannot use (…)? …
Rendered 0 of 1 files … — 1 error

A two-page site prints the block once, followed by Affected files: …/about.qmd, …/index.qmd.

• docs/ renders 271/271 pages with no Q-14-6 (nothing previously masked turns red).

Note: the hub-client vitest leg of cargo xtask verify is red locally under Node 26 (localStorage undefined), unrelated to this change — filed as bd-lh30hlvd.

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https://claude.ai/code/session_01Xwth8pr6ewSbExXRBNpQWY

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[cscheid]

CI finding on the first run: 20 hub-client WASM tests failed with Q-14-6 ("Can't find stylesheet to import vendor/rfs"). Four *.wasm.test.ts files load the WASM module by hand and render without calling setVfsCallbacks, so dart-sass could never resolve the Bootstrap bundle's imports. The old DEFAULT_CSS fallback masked that, and the tests only inspected HTML, so they passed against an unstyled page. This is the "surfacing currently-masked failures" risk from bd-36vmz7nk, realised in the test suite.

Fixed in c0a6230: a shared hub-client/src/test-utils/wasmSassVfs.ts (wireSassVfs) mirroring the production wiring in preview-runtime's wasmRenderer.ts, called from each file's beforeAll. npm run test:wasm is 133/133 locally.