feat(legacy): support definitions v2

### docs/common/external-definitions.md
@@ -120,6 +120,7 @@ signed by the CoSi algorithm.
 
 The format version is bumped on backward incompatible change.
 For versions 1 and 2, the data structure is identical but the number of necessary signatures is changed from 2 to 1.
+Trezor One (legacy firmware) accepts only format version 2 since version 1.14.2.
 
 The full format of the definition is as follows:
 

### legacy/firmware/ethereum_definitions.c
@@ -37,7 +37,7 @@
 
 typedef pb_byte_t proof_entry[SHA256_DIGEST_LENGTH];
 
-#define SIGNATURE_THRESHOLD 2
+#define SIGNATURE_THRESHOLD 1
 #define DEFS_PUBLIC_KEYS_COUNT 3
 
 const ed25519_public_key DEFS_PUBLIC_KEYS[DEFS_PUBLIC_KEYS_COUNT] = {

### legacy/firmware/ethereum_definitions_constants.h.mako
@@ -11,6 +11,6 @@
 
 #define MIN_DATA_VERSION ${defs_timestamp}
 #define FORMAT_VERSION_LENGTH 5
-#define FORMAT_VERSION (const pb_byte_t *)"trzd1"
+#define FORMAT_VERSION (const pb_byte_t *)"trzd2"
 
 #endif

### tests/definitions.py
@@ -40,7 +40,7 @@ def make_eth_token(
 
 def make_payload(
     magic: bytes = b"trzd",
-    format_version: bytes = b"1",
+    format_version: bytes = b"2",
     data_type: messages.DefinitionType = messages.DefinitionType.ETHEREUM_NETWORK,
     timestamp: int = 0xFFFF_FFFF,
     message: (

### tests/device_tests/ethereum/test_definitions_bad.py
@@ -109,6 +109,16 @@ def _make_display_format_payload(
     )
 
 
+def downgrade_to_v1(payload: bytes) -> bytes:
+    """Downgrade a v2 payload to v1 by swapping the version byte.
+
+    Formats 1 and 2 differ only in the version byte, so this is equivalent to
+    building the payload with format_version=b"1". Must be done before
+    signing, since the version byte is covered by the signature.
+    """
+    return b"trzd1" + payload[5:]
+
+
 def _cases(session: Session) -> list[tuple]:
     cases: list[tuple] = [
         (make_payload, _fails_network),
@@ -133,13 +143,24 @@ def test_mangled_signature(session: Session) -> None:
         check(session, payload + proof + bad_signature, "Invalid definition signature")
 
 
+@pytest.mark.models("core")
 def test_not_enough_signatures(session: Session) -> None:
+    # version 1 requires two signatures, one is not enough
     for make, check in _cases(session):
-        payload = make()
+        payload = downgrade_to_v1(make())
         proof, signature = sign_payload(payload, [], threshold=1)
         check(session, payload + proof + signature, "Invalid definition signature")
 
 
+@pytest.mark.models("legacy")
+def test_v1_format_version_rejected(session: Session) -> None:
+    # legacy firmware accepts only format version 2
+    for make, check in _cases(session):
+        payload = downgrade_to_v1(make())
+        proof, signature = sign_payload(payload, [])
+        check(session, payload + proof + signature, "Invalid definition")
+
+
 def test_missing_signature(session: Session) -> None:
     for make, check in _cases(session):
         payload = make()
@@ -183,7 +204,7 @@ def test_bad_prefix(session: Session) -> None:
     for make, check in _cases(session):
         payload = make()
         # mangle the magic, keep a valid version byte
-        payload = b"trze1" + payload[5:]
+        payload = b"trze" + payload[4:]
         proof, signature = sign_payload(payload, [])
         check(session, payload + proof + signature, "Invalid definition")
 

### tests/ui_tests/fixtures.json
@@ -450,13 +450,13 @@
 "T1B1_en_ethereum-test_definitions_bad.py::test_mangled_payload": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
 "T1B1_en_ethereum-test_definitions_bad.py::test_mangled_signature": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
 "T1B1_en_ethereum-test_definitions_bad.py::test_missing_signature": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
-"T1B1_en_ethereum-test_definitions_bad.py::test_not_enough_signatures": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
 "T1B1_en_ethereum-test_definitions_bad.py::test_outdated": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
 "T1B1_en_ethereum-test_definitions_bad.py::test_proof_length_mismatch": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
 "T1B1_en_ethereum-test_definitions_bad.py::test_protobuf_mismatch": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
 "T1B1_en_ethereum-test_definitions_bad.py::test_short_message": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
 "T1B1_en_ethereum-test_definitions_bad.py::test_trailing_garbage": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
 "T1B1_en_ethereum-test_definitions_bad.py::test_trimmed_proof": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
+"T1B1_en_ethereum-test_definitions_bad.py::test_v1_format_version_rejected": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
 "T1B1_en_ethereum-test_getaddress.py::test_getaddress[ETC]": "6c9f221da7bfc7c66f0332ef96130f3af353f4d5a8a57698093febaff9ec70ed",
 "T1B1_en_ethereum-test_getaddress.py::test_getaddress[Ledger Live legacy path]": "bc8a1f830af1231939970782cb96ead9d160cd1acd6e998e3e0e81910bcb9be2",
 "T1B1_en_ethereum-test_getaddress.py::test_getaddress[parameters0-result0]": "31ba4391758d58378afca4d00cd0b21dd0180bff35e3327c12b850e73e04e372",