ffi 0.7: #import c { #include / #source } via stdlib-path resolution

94/94 regression tests pass (+ffi-07-c-import-block).

Companion C helper lives only at
`library/vendors/sx_ffi_resolve_test/`. Critically NOT in
`sx/vendors/` (the sx repo root) and NOT in the importing
example's directory — so the `vendors/...` paths in this
example are findable solely via the stdlib search branch
(`<exe>/../../library`, `<exe>/../library`, `<exe>/library`).

That branch is the one the JNI insets bridge needs to reach
`library/vendors/sx_android_jni/sx_android_jni.c` without
forcing chess (or any consumer) to vendor an identically-named
copy. The test pins the resolution end-to-end:
  - #include  resolves; clang parses the .h; c_import.zig
    synthesizes #foreign fn decls for `sx_ffi_resolve_test_add` /
    `_mul`.
  - #source   resolves; the .c is compiled into the build's
    object list.
  - sx calls the synthesized decls and prints results.

tests/expected/ffi-07-c-import-block.exit

@@ -0,0 +1 @@
+0

tests/expected/ffi-07-c-import-block.txt

@@ -0,0 +1,2 @@
+add(3, 4)  = 7
+mul(6, 7)  = 42