feat(tools): land mixins from another Sass module where they are included (#1795)

A mixin included from another Sass module (`@use 'x'; @include x.m`) now
lands where it is included, as a same-file mixin already did: its
top-level weights meet the including rule's family, and its family
becomes that rule's, in the order Sass writes them out. Include sites
resolve through the existing `@use`/`@forward` scope resolution, the
definition Sass resolves is the one that runs, and an include inside a
mixin body resolves where that mixin runs. The header's "Not traced" list
keeps what stays out (positional arguments, content blocks placed by
another module's mixin, a name two `@import`ed files define).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/architecture/iptvnator-ui-guidelines.md

@@ -908,8 +908,10 @@ heavier faces would exceed the initial-bytes ratchet).
 - JetBrains Mono text stays at 500 or lighter, also where it is a fallback
   behind `ui-monospace` (only macOS resolves that). The check enforces this in
   any rule that sets the family, directly or through a variable, or inherits
-  it from an enclosing rule. It cannot see what a mono modifier class inherits
-  from its base rule; set `font-weight: 500` there.
+  it from an enclosing rule. A mixin's family and weights count where it is
+  included, from its own stylesheet module or another one. It cannot see what
+  a mono modifier class inherits from its base rule; set `font-weight: 500`
+  there.
 - Import whole `@fontsource/<family>/<weight>.css` files. The single-script
   files such as `cyrillic-600.css` have no `unicode-range`, so a Cyrillic-only
   face wins the weight match for Latin text in `Roboto, …` stacks and sends it