fix: size migration content SaveFlag enums like the live writer (v4.1.1)

[kamronbatman]

Problem

GenerateMigrationContentStruct emitted a single V{N}SaveFlag enum with no underlying type (so int), whatever the flag count. The live generator that wrote vN sizes its SaveFlag enums from the count: int up to 32 flags, ulong for 33-64, then one extra ulong enum (SaveFlag2, ...) per 64 flags.

• 33-64 flags: bit 31 emits 0x80000000 into an int enum, which fails with CS0266: Cannot implicitly convert type 'uint' to 'int'. This blocks ModernUO #2670, which needs to migrate from BaseCreature v23 (54 save-flagged fields).
• 65+ flags: the content struct reads one enum where the writer wrote several, so every field after the flags is misaligned.

Fix

The content struct now uses the same partitioning as the live side (Class.cs):

• V{N}SaveFlag, V{N}SaveFlag2, ... are : ulong once there are more than 32 flags, with values from GenerateEnumValueLong
• the constructor reads every flag enum before any field, as the writer does
• each field is tested against its own enum and variable

Output for 32 or fewer flags is byte-identical, and every existing snapshot still matches.

Readonly fields can't shift the bit numbering: Class.cs already leaves them out of the migration schema, the same way the live side skips them for flag bits.

Tests

• MigrationSaveFlagTests: a real round trip at 20, 40 and 70 flags. It compiles a v0 entity, serializes it with the live generator through a binary writer that sizes enums by their underlying type (as ModernUO does), then deserializes the bytes into v1 through MigrateFrom(V0Content). It checks every value (present values around bits 31/32 and 63/64, unset fields as null, a flagged bool) and that the whole stream was consumed. Before the fix, the 40 and 70 cases failed with the CS0266 above.
• Snapshots/MigrationSaveFlagsMultiEnum (66 flags) pins the emitted V0SaveFlag : ulong / V0SaveFlag2.
• Adds SourceGeneratorTestHelper.CompileAndLoad so tests can execute generated code. It shares its compile code with RunGeneratorAllOutputs.

dotnet test: 119 passed, 0 failed.

Release

Bumps Annotations, Generator and SchemaGenerator to 4.1.1.