fix(net,runtime): FA2 fix-round SHOULD-FIX -- fellowship mechanism parity, lookup reuse, router test, checkpoint defaults

Remaining SHOULD-FIX findings from the FA2 mechanism/blast reviews:

Mechanism SF-3/SF-4 -- RuntimeFellowshipState.ApplyUpdateFellow now ports
Fellowship::RecalculateEvenXPSplitting @0x005B92E0 (called from retail's
AddFellow/UpdateFellow/RemoveFellow on every upsert/removal, but never
from a full update -- that carries the server's own authoritative flag
verbatim, lane B 6.2) and Fellowship::AddFellow @0x005B9480's
locked/departed admission gate (a brand-new guid is refused while
_locked unless it appears in the 0x02BE field-8 _fellows_departed table
within 900s, @0x005B94A5). ApplyFullUpdate now stores update.Departed
instead of discarding it. A TimeProvider dependency (defaulting to
TimeProvider.System, matching the RuntimeCharacterOptionsState precedent)
makes the 900s grace window testable.

Mechanism SF-5 -- RuntimeAllegianceState's TryGetMember/TryGetPatron/
GetVassals now reuse ClientCommandResponses.AllegianceProfileLookups
(promoted private -> internal, AcDream.Runtime added to Core.Net's
InternalsVisibleTo) instead of re-implementing the retail walk a second
time.

Mechanism SF-6 -- RuntimeStateCheckpoint's Fellowship/Allegiance
parameters are no longer trailing-optional. `default(RuntimeFellowshipSnapshot)`/
`default(RuntimeAllegianceSnapshot)` zero-init Name/AllegianceName to
null, and C# does not allow a non-constant `new(...)` as an optional
parameter's default value (CS1736) even when the struct declares an
explicit parameterless constructor -- so the only way to guarantee a
non-null default was to make the parameters required. Both snapshot types
still gained an explicit parameterless constructor for callers that want
an empty-but-safe `new()`.

Blast SF-4 -- LiveSessionEventRouterTests gains
FellowshipQuit_RoutesSelfGuidToClearAndOtherGuidToRemove, wiring real
RuntimeFellowshipState/RuntimeAllegianceState owners through the one
production registration site and dispatching a real 0x00A3 envelope for
both a self-quit and an other-quit -- the one non-trivial lambda in the
slice (the self-guid source that decides "remove one member" vs "clear
the whole snapshot") was previously untested; every other router test
defaults Fellowship/Allegiance to null.

Blast SF-5 -- RuntimeFellowshipState.ResetSession dropped its disposed
guard to match the precedent its own doc comment names
(RuntimeInventoryState.ResetExternalContainer,
RuntimeCommunicationState.ResetNegotiatedChannels -- both bare delegations
with no disposal guard); the reset transaction is retryable and disposal
is terminal, so a throwing guard could never converge on retry.
RuntimeAllegianceState.ResetSession (new this fix round) matches the same
shape from the start.

Blast SF-7 -- IRuntimeAllegianceView.GetVassals' per-call List<> allocation
is now documented as an intentional exception to the file's "Snapshot +
TryGet*, no allocation" view convention (C# cannot yield-return from
inside a lock).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/AcDream.Runtime/GameRuntimeViews.cs

@@ -239,10 +239,21 @@ public readonly record struct RuntimeStateCheckpoint(
     RuntimeWorldEnvironmentOwnershipSnapshot EnvironmentOwnership,
     RuntimePortalSnapshot Portal,
     RuntimeWorldTransitOwnershipSnapshot TransitOwnership,
-    // Campaign FA slice FA2 (2026-08-12): default so every existing
-    // positional construction site (tests) compiles unchanged.
-    RuntimeFellowshipSnapshot Fellowship = default,
-    RuntimeAllegianceSnapshot Allegiance = default);
+    // Campaign FA slice FA2 (2026-08-12): originally trailing-optional
+    // (`= default`) so every existing positional construction site (tests)
+    // compiled unchanged. FA2 fix-round SHOULD-FIX 6 (blast review) made
+    // both parameters REQUIRED instead: `default(RuntimeFellowshipSnapshot)`/
+    // `default(RuntimeAllegianceSnapshot)` zero-init every field —
+    // including Name/AllegianceName to null, not string.Empty — and C#
+    // does not allow a non-constant expression (a real `new(...)` call) as
+    // an optional-parameter default, so there is no way to give a
+    // TRAILING-OPTIONAL parameter here a non-null default. Both snapshot
+    // types still declare an explicit parameterless constructor
+    // (`RuntimeFellowshipSnapshot()`/`RuntimeAllegianceSnapshot()`,
+    // GameRuntimeGameplayViews.cs) so callers that want an empty-but-safe
+    // snapshot can pass `new()` explicitly.
+    RuntimeFellowshipSnapshot Fellowship,
+    RuntimeAllegianceSnapshot Allegiance);
 
 public interface IGameRuntimeView
 {