craft-font-catalog: run the Unix font-folder tests only on Unix (#621)

* craft-font-catalog: run the Unix font-folder tests only on Unix

Five tests build an Os::Unix FontEnv from Unix paths (/home/me, <dir>/sys/fonts</dir>).
Windows doesn't treat those paths as absolute, so the tests failed there. Production code
is unaffected: on Windows FontEnv::from_process picks Os::Windows, whose folders never go
through the Unix path logic. FontEnv's docs now say its paths follow the host's path rules.

The font-store workflow, which already triggers on crates/font-catalog, now also runs
craft-font-catalog's clippy and tests on Linux and Windows.

Refs #619

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

* craft-font-catalog: keep the store override and no-store tests on Windows

These assertions don't depend on Unix path rules, so they now sit in their own
ungated test.

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

* craft-font-catalog: verify store override precedence

Exercise the override with platform defaults present on every OS variant.

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

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/font-store.yml

@@ -29,6 +29,8 @@ jobs:
       - run: cargo fmt --check
       - run: cargo clippy -p craft-font-store --all-targets --locked -- -D warnings
       - run: cargo test -p craft-font-store --locked
+      - run: cargo clippy -p craft-font-catalog --all-targets --locked -- -D warnings
+      - run: cargo test -p craft-font-catalog --locked
       - name: Check the web build
         if: runner.os == 'Linux'
         run: cargo check -p craft-font-store --target wasm32-unknown-unknown --locked