Support brand.yml font weight ranges; make invalid weights a Q-14-8 error (bd-5fseopxy)

[cscheid]

Fixes bd-5fseopxy.

Problem

brand.yml allows a variable-font weight range written as a string (weight: 400..700). Quarto 2 parsed it as an unknown keyword and then:

• google: enumerate_weights did weight_name_to_number(s).unwrap_or(400) — the @import requested only weight 400, bold fell back to synthetic bold, no diagnostic.
• file (files[].weight: 300..800): the string was written verbatim into @font-face as font-weight: 300..800;, which grass rejects — the whole theme silently fell back to the 7 KB DEFAULT_CSS.
• slot (headings: {weight: 500..700}): same verbatim passthrough into $headings-font-weight: 500..700 !default;, same silent fallback.

Any other unrecognised weight string (Bold, semibold, bolder) also became a silent 400. Quarto 1 fails hard here (Unknown font weight).

Change

• quarto-brand: BrandFontWeight::Range(BrandFontWeightRange { min, max }), deserialised from exactly <digits>..<digits> (anything else stays Name so it can be reported verbatim). New Brand::validate() (validate.rs) checks every weight: under typography and returns BrandError::InvalidFontWeight { path, value, reason } with a BrandPath such as typography.fonts[0].files[1].weight. Rules: numbers in 100..=900; keywords from the closed Q1 table (weight_name_to_number now lives here); ranges numeric only, min <= max, allowed on fonts[] entries but not on slots.
• quarto-sass emission: google emits the wght@400..700 axis (with italics: ital,wght@0,400..700;1,400..700); bunny expands a range to discrete weights (400,500,600,700 — Bunny has no range syntax and silently serves 400 for 400..700, checked against fonts.bunny.net); @font-face emits font-weight: 300 800. Unknown keywords and slot ranges are Err, never a fallback.
• quarto-sass location: load_split_brand validates after parsing and, on failure, re-parses the brand text with quarto_yaml::parse_file and walks the path to the offending scalar. Inline brand: blocks are validated at extraction time while the ConfigValue tree (with per-node spans) is in scope. New SassError::InvalidBrandFontWeight { path, value, reason, location, brand_file }.
• quarto-core: new Q-14-8 "Invalid brand font weight" in theme_diagnostic; the error's brand_file is added as a bind candidate so the snippet renders from _brand.yml. The two stage_error("brand resolution: …") sites now route through theme_diagnostic, so brand-shape errors render with Q-14-1 too.
• Catalog entry, docs/errors/theme/Q-14-8.qmd, sidebar entry, and the weight section of docs/guides/authoring/brand.qmd.

What the user sees now

$ q2 render repro-slot-only
Error: [Q-14-8] Invalid brand font weight
   ╭─[ …/_brand.yml:9:13 ]
 9 │     weight: 500..700
   │             ────┬───
   │                 ╰───── `500..700` at `typography.headings.weight` is not a font weight Quarto
                            understands: a typography slot takes a single weight, not a range; …
ℹ Use a number from 100 to 900, a keyword such as `bold` or `semi-bold`, a list of those, or —
  on `typography.fonts` entries — a numeric range such as `400..700`?

and for the valid forms the compiled CSS contains

@import url("https://fonts.googleapis.com/css2?family=EB+Garamond:ital,wght@0,400..700;1,400..700&display=swap")
@font-face{font-family:"Local Var";src:url("LocalVar-VariableFont_wght.woff2");font-weight:300 800;font-style:normal}

Tests (written first, observed failing)

• quarto-brand/tests/integration/font_weight_test.rs — 24 tests: parsing of the range variant, the keyword table, every accepted shape, and each rejection (reversed, out of range, keyword ends, half-open, unknown keyword, list index, file-entry path, slot range, every slot name).
• quarto-sass — emission (google axis, italics, bunny expansion incl. non-round ends, file pair, keyword list regression guard, defensive Errs), grass compile of the range brand, and the location tests (path form: span keyed by file_id_for_filename, covering exactly the scalar; inline form: the ConfigValue node's span).
• quarto-core theme_diagnostic — Q-14-8 renders with code/path/value/reason and a snippet from _brand.yml with no caller-supplied candidates; span-less form; catalog registration.
• quarto/tests/integration/brand_font_weight.rs — five CLI end-to-end tests through the q2 binary.

Notes for review

• Design decisions and the investigation record: claude-notes/plans/2026-09-08-brand-font-weight-ranges.md; repro fixtures under claude-notes/plans/brand-font-weight-ranges-investigation/.
• Coordination with bd-jsvetdea (theme compile errors, in flight): that work claims Q-14-6/Q-14-7 and edits theme_diagnostic.rs and quarto-sass/src/error.rs; this PR adds one arm and one variant to each. Small, but expect a trivial merge in whichever lands second.
• source: file is touched only on the font-weight line of the @font-face block, deliberately; the suspected deeper file-handling bugs are being planned separately.
• Default weights when weight: is absent stay at 400;700 (Q1 parity); the brand.yml reference requests all nine — split off as bd-b1rrnzp1.
• Verification: cargo xtask verify --skip-hub-tests green through Rust build, clippy, workspace tests, ts-packages, and the hub-client WASM build. The hub-client vitest leg is skipped for the known Node 26 localStorage environment failure (bd-lh30hlvd). The preview-renderer integration suite failed once on a stale local node_modules (KaTeX 0.17 installed vs 0.18.4 pinned) and passes after npm install.

🤖 Generated with Claude Code

https://claude.ai/code/session_01NE1UEmUUagZwZ5APMZYfra

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