Add managed trash container state and event APIs

[diffuin]

Closes #240.

Summary

• add a managed TrashContainer wrapper for existing native containers
• expose capacity, current and normalized level, immutable content snapshots, eligibility, direct lookup, and scene enumeration
• forward trash-added and level-changed events with duplicate and cross-wrapper unsubscribe semantics
• gate TryBagTrash on server authority and native eligibility, then use the native replicated bagging path
• keep custom-container persistence and replacement replication out of scope

Review repairs

• track native event registrations per native container instead of per wrapper, preventing abandoned wrappers from retaining listeners and allowing removal through a separately obtained wrapper
• preserve one-at-a-time removal for duplicate subscriptions
• use runtime-native Unity delegate identity on IL2CPP while retaining the established Mono event bridge
• add managed API-shape and immutability contract coverage

Validation

• MonoMelon build: 0 warnings, 0 errors
• Mono contract suite: 662 passed
• Il2CppMelon build: 0 warnings, 0 errors
• IL2CPP contract suite: 648 passed
• DocFX: succeeded; public API documentation coverage 81.84% (minimum 80%)
• git diff --check: clean

In-game Mono

Passed on Schedule I 0.4.6f13 Alternate using a copied disposable completed save. Verified direct lookup, no hierarchy fallback, active/inactive enumeration, duplicate and cross-wrapper removal for both events, immutable snapshots, live off-server rejection, authoritative bagging, level-change forwarding, and native clearing.

In-game IL2CPP

Passed on Schedule I 0.4.6f13 using a copied disposable completed save. Verified direct lookup, no hierarchy fallback, active/inactive enumeration, duplicate and cross-wrapper removal for both events, immutable snapshots, authoritative bagging, level-change forwarding, and native clearing. The non-server branch is source-gated by InstanceFinder.IsServer; the live non-server rejection was exercised in Mono before world load.

Both game installations were restored after testing, no probe remained installed, and the source save fixtures were not modified.

Compatibility

Additive public API only. No existing source, binary, save, or network payload contract is changed. Bagging delegates to the native server-authoritative implementation.