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/platforms/test_existence.yml

@@ -0,0 +1,22 @@
+platform: TestExistence
+verification_mode: existence
+base_destination: system
+systems:
+  test-system:
+    files:
+    - name: required_present.bin
+      destination: required_present.bin
+      required: true
+      sha1: placeholder
+    - name: required_missing.bin
+      destination: required_missing.bin
+      required: true
+      sha1: "0000000000000000000000000000000000000000"
+    - name: optional_present.bin
+      destination: optional_present.bin
+      required: false
+      sha1: placeholder
+    - name: optional_missing.bin
+      destination: optional_missing.bin
+      required: false
+      sha1: "0000000000000000000000000000000000000001"