fix(ci): make terraphim_service optional in github_runner, restore Rust Build

- Make terraphim_service optional behind github-runner feature
- Gate WorkflowParser behind github-runner feature
- Gate parser.rs test module behind github-runner feature
- Remove github_runner_server from exclude (feature-gated)
- Revert Rust Build continue-on-error (github_runner now compiles)

Author: Test User

.github/workflows/ci-main.yml

@@ -147,9 +147,6 @@ jobs:
   # .docs/walkthrough-ci-build-pipeline.md and the two ADRs it links.
   rust-build:
     name: Rust Build (${{ matrix.target }})
-    # Soft-fail: terraphim_service registry crate has private-method bug
-    # that blocks workspace compilation. Fix: republish terraphim_service.
-    continue-on-error: true
     runs-on: [self-hosted, bigbox]
     timeout-minutes: 30
     needs: [setup, frontend-build, rust-format]