ci(e2e): build the standalone Zig FFI surface (no cross-org bridge)

The E2E/Bench jobs ran the default build.zig, which compiles the full
Idris2->RefC->Zig pipeline and pulls in idris2_zig_ffi via a Zig .path
dependency on the private, sibling-checkout-only repo
nextgen-languages/language-bridges. CI never provides that path, so
`zig build` failed at dependency resolution (the ~19s failure, once the
toolchain install was fixed).

Point both jobs at build_standalone.zig, which builds src/main.zig only
(the pure-Zig C-ABI surface: proven_init, proven_path_has_traversal,
proven_header_has_crlf, ...) with no Idris/RefC or external dependency.
Verified src/main.zig imports only std + builtin, so it compiles
standalone. Output is libproven_ffi.a (glob updated accordingly).

Scope note: this exercises the Zig FFI build + unit tests, not the full
Idris->Zig integration. The full pipeline is deferred until the private
language-bridges repo is provisioned (submodule + CI credential).

https://claude.ai/code/session_01MN5vzRR4MK2dkDNaHqqRDy

Author: claude

.github/workflows/e2e.yml

@@ -47,14 +47,18 @@ jobs:
         with:
           version: '0.15.2'
 
-      - name: Build FFI
-        run: cd ffi/zig && zig build
+      # Build the standalone pure-Zig FFI surface (src/main.zig only) -- no
+      # Idris2 RefC / idris2_zig_ffi dependency. The full Idris->RefC->Zig
+      # pipeline needs the private nextgen-languages/language-bridges repo,
+      # which CI cannot fetch as a path dependency; see PR #149.
+      - name: Build FFI (standalone Zig surface)
+        run: cd ffi/zig && zig build --build-file build_standalone.zig
 
-      - name: Run FFI integration tests
-        run: cd ffi/zig && zig build test
+      - name: Run FFI tests (standalone)
+        run: cd ffi/zig && zig build test --build-file build_standalone.zig
 
       - name: Verify library output
-        run: ls -la ffi/zig/zig-out/lib/libproven.* || echo "Library output check"
+        run: ls -la ffi/zig/zig-out/lib/libproven_ffi.* || echo "Library output check"
 
   safety-aspects:
     name: Aspect — Safety Invariants
@@ -159,7 +163,7 @@ jobs:
           version: '0.15.2'
 
       - name: Build and bench FFI
-        run: cd ffi/zig && zig build bench 2>&1 | tee /tmp/bench-results.txt || echo "No bench target yet"
+        run: cd ffi/zig && zig build bench --build-file build_standalone.zig 2>&1 | tee /tmp/bench-results.txt || echo "No bench target yet"
 
       - name: Upload benchmark results
         if: always()