Run the adapter's feature-gated configuration in CI

The `memory-git` feature added alongside the lifted diff family gates that
family, and the test asserting it is *withheld* without the feature is
`#[cfg(not(feature = "memory-git"))]`. Both existing jobs — `--all-features`
and default — compile that test out, so it was checked by nothing: a
feature-gated test whose default fate is to be built by one job and executed
by none.

Adds the configuration as its own step, and asserts the property the feature
exists for: `cargo tree --no-default-features` must link no `git2` or
`libgit2-sys`. A guard rather than a comment, because "this feature keeps the
native build out of the graph" is a claim that silently stops being true the
first time a dependency picks it up transitively.

Verified: `diff_is_withheld_when_the_snapshot_store_is_compiled_out` appears in
`--no-default-features -- --list` and is absent from the `--all-features`
listing, which is what "running nowhere" looked like.

Refs #18 (§C3, §E2)

Author: YellowSnnowmann

.github/workflows/ci.yml

@@ -49,6 +49,30 @@ jobs:
       - name: Test default features
         run: cargo test
 
+      # The adapter's `memory-git` feature gates the diff family, and the test
+      # that the family is *withheld* without it is `#[cfg(not(feature =
+      # "memory-git"))]`. Both the `--all-features` and default runs above
+      # compile that test out, so without this step it would be checked by
+      # nothing — a feature-gated test whose default fate is to be built by one
+      # job and executed by none.
+      #
+      # This is also the configuration that keeps the promise the feature
+      # exists for: no `git2` / `libgit2-sys` in the graph.
+      - name: Lint and test the adapter without its optional engine features
+        run: |
+          cargo clippy -p tinymemory-tinycortex --all-targets --no-default-features -- -D warnings
+          cargo test -p tinymemory-tinycortex --no-default-features
+
+      - name: Assert the default adapter build links no native git
+        run: |
+          linked="$(cargo tree -p tinymemory-tinycortex --no-default-features \
+            -e normal --prefix none | grep -cE '^(git2|libgit2-sys)' || true)"
+          if [ "$linked" -ne 0 ]; then
+            echo "the default adapter build linked $linked native-git crate(s);" >&2
+            echo "the memory-git feature exists to keep them out" >&2
+            exit 1
+          fi
+
   # The module crate is its own workspace root (see the `exclude` note in the
   # root Cargo.toml), so NONE of the steps above touch it: `--all-targets`,
   # `--all-features` and `--workspace` all stop at the workspace boundary and