feat: collection helpers return whether they changed the collection (v4.3.0)

[kamronbatman]

Problem

The generated collection helpers return void, so a caller that needs to know whether a removal happened cannot use them. It falls back to ISerializableExtensions.Remove(list, value), or a Contains check followed by RemoveFromX (two lookups). ModernUO's Corpse.OnItemLifted is an example: it removes the lifted item from EquipItems and branches on whether it was worn gear.

Changes

Return types now mirror the BCL. A helper that can be a no-op returns whether it changed anything:

Helper	Before	After
RemoveFromX (list, set, dictionary)	void	bool, like Remove
AddToX on a set	void	bool, like HashSet<T>.Add
AddToX on a dictionary	void	bool, like TryAdd
AddToX on a list, InsertIntoX, RemoveFromXAt, ReplaceInX, ClearX	void	unchanged

• MarkDirty still fires only on an actual change, as in 4.2.0.
• Without a dirty target (SG3019), the helpers return the mutation's result directly.
• The ContainsKey + Add fallback for dictionaries without TryAdd returns bool too. No serializable type reaches that branch today, since only Dictionary<,> has a migration rule.
• Generator, SchemaGenerator and Annotations bumped to 4.3.0. README install snippet updated, plus a line on the return values.

Compatibility

Callers that ignore the result keep compiling. A helper used as a method group for an Action<T> (e.g. list.ForEach(RemoveFromX)) would no longer compile. ModernUO has no such use.

Tests

• DataStructureMethodTests.Helpers_TolerateNullAndMarkDirtyOnlyOnChange now asserts each helper's return value next to the dirty count: null collections, duplicates, missing entries, and successful adds and removes for lists, sets and dictionaries.
• New Helpers_ReportChangesWithoutADirtyTarget runs the helpers on an entity with no dirty target.
• 9 snapshot files changed, all in helper signatures and bodies. All 130 tests pass.

After merge

Run the Build and Publish NuGet Packages workflow. A ModernUO follow-up then bumps to 4.3.0 and replaces ISerializableExtensions.Remove call sites with the generated helpers, starting with Corpse.OnItemLifted.