[WC-3392] Fix duplicate color variant imports in Atlas Core

[rahmanunver]

Problem

_color-variants.scss was emitted three times into atlas_core.css, and a fourth time into the app's theme.compiled.css. Its output is a ~18 KB :root block plus shade classes, byte-identical on every emission (in atlas_core.css at output lines 481 / 859 / 1269).

Cause: a core -> theme -> core import round trip. Legacy @import is a textual paste and never dedupes.

themesource/atlas_core/web/main.scss
├─ :6  @import "color-variants"                          → emission 1
├─ :7  @import ../../../theme/web/custom-variables
│        └─ custom-variables.scss:603
│             @import ../../themesource/atlas_core/web/variables   ← the round trip
│               └─ _variables.scss:7  @import "color-variants"  → emission 2
└─ :8  @import "variables"
         └─ _variables.scss:7  @import "color-variants"         → emission 3

theme/web/main.scss                    (separate bundle → theme.compiled.css)
└─ :1  @import "custom-variables"
         └─ custom-variables.scss:603  (same round trip)
              └─ _variables.scss:7  @import "color-variants"    → emission 4

custom-variables.scss lives in theme/ — it is the app author's customization template. Reaching back down into themesource/atlas_core/ is what closed the loop, and it did so in both bundles.

Solution

One line removed, at the root:

• Deleted the upward import at custom-variables.scss:603.
• Removed the now-redundant direct @import "color-variants" from atlas_core/web/main.scss — @import "variables" on the next line already pulls it in.
• Added !default to $brand-colors and $lightness-steps so a future re-import cannot clobber a downstream override.

Nothing was relocated. theme/web/main.scss is untouched: theme-dark and theme-neutral use only CSS custom properties, no Sass variables, so the theme bundle never needed variables at all. Legacy Sass-variable support for modules is unaffected — it still comes from atlas_core/web/main.scss's own @import "variables".

Result

Bundle	Before	After	Δ
atlas_core.css	835,034	800,029	−34 KB
theme.compiled.css (theme/web/)	22,737	4,721	−18 KB
atlas_web_content.css	13,098	13,098	—

~52 KB total. The removed declarations are byte-identical to the surviving copy in atlas_core.css (verified by comparing the extracted blocks), so nothing is lost at runtime.

validateSass passes. Note it only compiles the two themesource entries, not theme/web/main.scss — which is why the fourth emission went unnoticed.

Note on the diff

The pre-commit hook (pretty-quick) reformats custom-variables.scss wholesale on any edit — it had been committed with 2-space indentation. That churn is isolated in its own commit (style: format custom-variables.scss with prettier, whitespace plus two hex literals lowercased). The actual fix is 6 changed lines across 3 files.