feat(brand): add _brand.yml support across the render pipeline

New `quarto-brand` crate owns the typed `_brand.yml` data model (color
palette + named theme colors, typography slots, logo, defaults) with
serde-driven validation via `deny_unknown_fields`. Ports Q1's
`Brand.getColor` palette/named-color resolution (with cycle detection),
monospace inline/block merging, and logo path resolution.

`quarto-sass::brand_layer` ports Q1's `brandColorLayer` /
`brandDefaultsBootstrapLayer` / `brandTypographyLayer` to produce
`SassLayer`s. Font-family values are quoted (one intentional Q2
improvement over Q1's bare emission, which is fragile for multi-word
names).

`ThemeSpec::Brand` is a new position marker for the literal token
`brand` in the `theme:` array — same semantics as Q1's
`format-html-scss.ts:216`. `ThemeConfig` grows an unresolved
`brand_ref: Option<BrandRef>` (path or inline block) populated by
`from_config_value` (pure, no I/O); a sync `ThemeConfig::resolve(&dyn
SystemRuntime, base_dir)` reads the file and produces a
`ResolvedThemeConfig` with a typed `Brand`. The split keeps the parsing
pure and routes all brand I/O through one site
(`CompileThemeCssStage::run`), so native CLI and WASM hub-client share
the code path via `SystemRuntime`.

`ThemeContext::with_brand` carries the resolved `Brand` into
`process_theme_specs`, which expands `ThemeSpec::Brand` into the
brand-derived layers. Cache key in `CompileThemeCssStage` hashes the
brand's YAML serialization so brand edits invalidate.

Coverage:
- 31 unit tests in `quarto-brand` (parse, color, typography, logo)
- 14 string-search + 6 grass-compile parity tests for brand_layer
- 12 ThemeConfig integration tests
- 2 end-to-end tests in `quarto-core/tests/brand_render.rs` that drive
  the real `render_to_file` API against project + brand and brand-only
  fixtures
- Manual smoke: `q2 render` against project+theme+brand, brand-only,
  and explicit-position fixtures. Verified --bs-primary-rgb /
  --brand-* / --bs-body-color / --bs-body-font-family in compiled CSS.

Workspace tests: 9292/9292 pass. `cargo xtask verify --skip-hub-build`
green.

Out of scope (filed as follow-ups): light/dark brand pairs (bd-v5z8w),
brand favicon hookup (bd-1elkd), navbar brand-image (bd-hp3tx), typst
brand (bd-dsco4), `quarto use brand` scaffolding (bd-1vlw8), wire into
future YAML validator (bd-q1fyw), scss-analysis annotations
(bd-u67gw), inline brand in single-file frontmatter (bd-rwxa0),
hub-client browser verify (bd-wjg4h).

Plan: claude-notes/plans/2026-05-20-brand-yml-support.md
Docs: docs/guide/themes/brand.qmd

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: cscheid

.beads/issues.jsonl

@@ -156,6 +156,9 @@ path = "./crates/quarto-lsp-core"
 [workspace.dependencies.quarto-lsp]
 path = "./crates/quarto-lsp"
 
+[workspace.dependencies.quarto-brand]
+path = "./crates/quarto-brand"
+
 [workspace.dependencies.quarto-sass]
 path = "./crates/quarto-sass"