[desig9stein]

Closes #641.

Draft for review of the approach proposed in #641 (comment).

Problem

Sass 1.95+ deprecates the legacy if() function, so every consumer compile on a current Sass prints [if-function] warnings from sass/. Compiling the e2e theme with sass-embedded 1.107.1 on master prints 74 of them. The repo pins sass-embedded ~1.92.1, which predates the deprecation, so CI never saw them.

The new if(sass($c): $a; else: $b) syntax is not an option while Prettier formats sass/** in CI:

• 3.8.1 (the lockfile pin) and 3.8.2 add trailing commas that break the compile (Undefined operation "(21,) < 1.07").
• 3.8.3 to 3.8.5 insert a space before every ; that follows a ) or a string.
• 3.9.x collapses one-element lists such as (background,) into (background) in 36 untouched files, which changes their Sass type.

Change

Sass

• Every conditional in sass/ is an @if/@else block or an or/and expression. No if() call is left, including in the tile-manager theme from feat(tile-manager): add theme and MCP metadata #667.
• _optional-shades(), _default-variant() and bem--not-selector() replace the nested if() calls that repeated the same branch.
• $bem--sep-elem, $bem--sep-mod and $bem--sep-mod-val are !default. Before, they read meta.variable-exists(), which is always false inside a module, so @use ... with (...) could not set them. They keep the same values.
• The supported Sass range does not change. The rewrite uses nothing newer than Sass 1.92.

Dependencies

• sass-embedded is ~1.107.1 in packages/theming, packages/mcp and packages/preview. In theming and mcp it is a runtime dependency, because the MCP server compiles CSS with it, so consumers install 1.107.1 instead of 1.92.1.

Tests

• tests/deprecations.test.ts compiles the e2e theme with a logger and fails on any deprecation warning.
• tests/_themes.spec.scss uses an _fallback() helper instead of the 16 copies of the same if().

Tests

• npm run lint: 0 errors. npm run build: passes with 0 deprecation warnings. npm test: 42 files, 1522 tests pass.

• deprecations.test.ts fails on master's sass/ with 74 [if-function] warnings and passes on this branch.

• The compiled output does not change. Each comparison is byte-identical:

  Compared	Output
  Build output (dist/json, dist/tailwind, e2e theme.css), master vs branch on the same compiler	identical
  All 69 component themes on all 8 schemas plus the 8 palettes (1.7 MB of CSS), master on 1.92.1 vs branch on 1.107.1	identical
  The same file, branch on 1.92.1 vs branch on 1.107.1	identical

Known limit

deprecations.test.ts compiles only the e2e theme, which covers one component theme. The all-themes compile above shows no deprecations today, but a future deprecation in a component theme outside the e2e theme would not fail CI. One option is to pass fatalDeprecations to the sass-true run in tests/sass.test.ts, so every spec fails on a deprecation. I left that out to keep this PR to the migration.

Notes

• A stylelint autofix also limits the syntax: scss/at-if-no-parentheses --fix strips the outer parentheses of @if ($a and $b) or ($c and $d), which leaves invalid Sass. The one mixed condition in _generator.scss stays unparenthesized, which follows and binding tighter than or.