feat: YAML integration tests with fixtures (35 new, 118 total)

tests/fixtures/ with synthetic platforms, emulators, bios files:
- test_existence.yml: required/optional present/missing
- test_md5.yml: correct/wrong hash, zipped_file (3 cases),
  multi-hash Recalbox, truncated MD5 Batocera, dedup
- test_inherit.yml: YAML inheritance
- _shared.yml: shared groups
- emulator profiles with aliases, standalone mode, alias type

tests/test_integration.py covers:
- existence/md5 mode severity counts
- zipped_file inner ROM verification (3 cases)
- Recalbox multi-hash + Batocera truncated MD5
- worst-status aggregation per destination
- YAML inheritance
- cross-reference: undeclared, standalone skipped, alias skipped
- alias resolution
- pack consistency (verify == pack counts)
- shared groups parsing

All tests use real files, real hashes, real functions, no mocking.

tests/fixtures/emulators/test_emu_alias_only.yml

@@ -0,0 +1,5 @@
+emulator: TestAlias
+type: alias
+alias_of: test_emu_with_aliases
+systems: [test-system]
+files: []

tests/fixtures/emulators/test_emu_with_aliases.yml

@@ -0,0 +1,12 @@
+emulator: TestEmulator
+type: standalone + libretro
+systems: [test-system]
+files:
+  - name: correct_hash.bin
+    required: true
+    aliases: [alt1.bin, alt2.bin]
+  - name: optional_standalone.rom
+    required: false
+    mode: standalone
+  - name: undeclared.bin
+    required: true