Close two holes in the test guards

[saworbit]

Summary

Two holes in the test guards, both P3.

• tests/test_level_root_shims.gd reads a script built from source only when it is one """ block. It now refuses any other form. The two stand-ins that hid as joined strings, in the validation and entity I/O vocabulary tests, now use the real LevelRoot. Four other built scripts moved into blocks; the NavigationMesh mock in the bake tests builds its source from the caller's names, so it carries the marker. The tooltip stub uses follow_grid in place of grid_snap, which is a level setting.
• tests/test_vibe_scenarios_load.gd now reads each root.name( and root.member.name( call in the vibe harness and asks a real level whether it has that name. Renaming validate_convexity at geometry.gd:76 fails it and names the file, line and method. GUT time is about 4s.

The new check found two scenarios that had stopped asking their question behind a has_method or has_signal check:

• build-outdoors called root.create_default_spawn(), which lives on the spawn system, and looked for the spawn by a meta a DraftEntity does not carry. It now makes one and finds it with get_active_spawn().
• save-as listened for hflevel_saved, which is hflevel_save_completed now. It counted no saves, so its check could never fire. It now counts three against three files.

Before / After Behavior

• Before: a stand-in joined with + passed the guard, and a renamed level method in a scenario passed CI.
• After: both fail CI and say where.

Related Issue

Fixes #957
Fixes #958

Checks

• python tools/run_local_checks.py passes (21 passed, 4 not run here)
• Each changed test file passes alone with -gselect, and the leak gate is clean on those logs
• python tools/vibe/run_vibe.py build-outdoors save-as is clean before and after
• run_vibe.py --selftest passes
• CHANGELOG, CONTRIBUTING, DEVELOPMENT and the vibe README updated
• git diff --check is clean