Build every tests/apply fixture through apply_fixture: 9 suites in the apply binary re-write the manifest, blob, package and event-lookup helpers it already has #1941

[agent] Filed by the scheduled architecture audit (audit-cli-tests). Register: register comment.

Kind: refactor. Register ID: T51. Follows the closed tracking issue #824 (shared CLI test helpers), scoped to one test binary.

Problem

tests/apply/apply_fixture.rs (L28-L94) has run_apply, patch_record, one_file, write_manifest, stage_blob, install_npm_pkg, write_root_package_json, install_gem and write_gem_patch. Other modules of the same apply test binary write their own:

• manifest + blob writers: apply_network.rs L22-L94,`` apply_invariants.rs L29-L66 (+ L156-L210, L540-L595), bun_global_store.rs L90-L120,```` pnpm_global_virtual_store.rs L93-L114,````` cargo_lock_source.rs L25-L46,`````` check_verifies_installed_tree.rs L31-L68``````` (+ L215-L247), lockfile_only_skip.rs L22-L72 (same names as apply_fixture's);
• the rack@3.1.0 gem patch three more times: gem_stale_bundler_plugin.rs L40-L69,````````` in_process_gem_config_warning.rs L38-L76,`````````` in_process_gem_fallback_home.rs L32-L91,``````````` plus a fourth fallback-home fixture in apply_render.rs L304-L403;````````````
• the event-by-purl lookup, identical, in bun_global_store.rs L124-L131,````````````` pnpm_global_virtual_store.rs L116-L123,`````````````` lockfile_only_skip.rs L150-L157,``````````````` cargo_lock_source.rs L137-L144 (common/envelope.rs has no by-purl finder);
• link_dir identical in bun_global_store.rs L22-L37 and pnpm_global_virtual_store.rs L23-L38;
• 13 explicit ("SOCKET_TELEMETRY_DISABLED","1") env pairs that hermetic::command already forces (hermetic.rs L72-L76).

Drift: exportedAt 2024 vs common::write_minimal_manifest's 2026; index.js vs package/index.js; three gem copies hand-format the JSON; gem_stale_bundler_plugin.rs L75-L90```````````````` spawns without the Extra::Bundle scrub every other gem runner applies, so an ambient `BUNDLE_*` reaches that child. `apply_fixture::write_manifest` and `common::write_minimal_manifest` (common/mod.rs L320-L386) are themselves two writers for one shape.

Proposed change

1. Make apply_fixture the only fixture module of the apply binary: delete the local copies above and call its helpers; move the fallback-home human test from apply_render into in_process_gem_fallback_home.
2. Add event_for(&Value, purl) -> &Value to common/envelope.rs and link_dir to apply_fixture.
3. Implement apply_fixture::write_manifest/stage_blob on top of common::write_minimal_manifest/write_blob (add a records parameter there) so there is one manifest writer.
4. Drop the redundant telemetry env pairs.

Size

~11 files in tests/apply/ plus common/envelope.rs; roughly −450/+60 lines. No product code.

Acceptance criteria

• cargo test -p socket-patch-cli --test apply passes with the same test count.
• No fn write_manifest, fn stage_blob, fn event( or fn link_dir outside apply_fixture.rs / common/ in tests/apply/.
• gem_stale_bundler_plugin's child gets the Extra::Bundle scrub.