Fix RegisterSyncField(Type, string) to preserve caller targetType (#880)

[sviyh]

Summary

Fixes #880.

MP.RegisterSyncField(Type, string) currently routes through RegisterSyncField(FieldInfo), which keys everything off FieldInfo.ReflectedType. Because AccessTools.Field walks the inheritance chain via Type.GetField, the returned FieldInfo for a private field declared on a base class comes back with ReflectedType collapsed to the base type — silently discarding the concrete Type the caller passed in.

Downstream, SyncField stores that base targetType (SyncField.cs L24-30), and DoSync serializes target as the base type (L47-52). Any SyncWorker registered against the concrete subclass then fails to match, because the wire format now references the base.

Concrete case that surfaced this: Gizmo_Slider.targetValuePct is declared on the base Gizmo_Slider. A compat patch calling MP.RegisterSyncField(concreteSubclass, \"targetValuePct\") ended up registered against Gizmo_Slider instead, breaking the subclass SyncWorker lookup. The Vanilla Gravship Expanded compat patch currently works around this with a reflection SetValue on the private targetType field (rwmt/Multiplayer-Compatibility#577); that workaround can be removed once this ships.

Fix

• RegisterSyncField(Type, string) now looks up the FieldInfo only to validate existence and branch on static, but passes the caller's targetType straight through to Sync.Field and the memberPath key. The internal Sync.Field(Type, string, string) path already accepted an arbitrary Type — it just wasn't reachable from the public compat API.
• RegisterSyncField(FieldInfo) is collapsed into a one-line delegate to (field.ReflectedType, field.Name), preserving its prior behavior exactly and eliminating the duplicated switch.
• Style matches the surrounding file: explicit FieldInfo declaration, ?? throw new Exception(...) as in Sync.Method, K&R braces.

No behavior change for callers whose targetType equals the declaring type (the common case), or for any caller of the FieldInfo overload.

Test plan

• dotnet build Source/Client/Multiplayer.csproj -c Release — 0 errors
• In-game: VGE oxygen regulator slider syncs across clients without the compat-side SetValue workaround
• Regression spot-check of existing RegisterSyncField(Type, string) callers in Multiplayer-Compatibility (Pharmacist, AnimalTab, SRTS Expanded, VEF autocast) — where targetType already equals the declaring type, behavior is unchanged
• Regression spot-check of RegisterSyncField(FieldInfo) callers (CommonSense, VanillaHairExpanded) — routes through the same path as before via ReflectedType