docs: update serialization docs and skills for generator v4 (#2588)

## Summary

Brings every serialization-related doc, skill, and the CLAUDE.md rule in line with generator **v4** (adopted in #2586/#2587). No code changes.

**Updated surface, everywhere it was referenced:**
- `[SerializableFieldSaveFlag(order)]` / `[SerializableFieldDefault(order)]` → `[SaveFlag(nameof(Should), nameof(Default))]` on the field (second method optional).
- `[TimerDrift]` + `[DeserializeTimerField(order)]` → `[DeserializeTimer(nameof(Method), wallClock)]` on the field, with the anchored-time semantics spelled out: drifting by default (downtime preserves the remaining delay, idle saves byte-stable), `wallClock: true` for absolute deadlines, restart method invoked **only when a timer was running** (no sentinel), and the timer `MigrateFrom` pattern (`XxxNext`/`XxxDelay`) for wire-format changes.
- New `[SerializableField]` documentation: the real signature (the documented `saveIf` parameter never existed) plus the setter hooks — `allowFieldChange` (`bool Method(ref T value)`: coerce/veto before assignment) and `fieldChanged` (`void Method(T oldValue, T newValue)` after) — with the generated pipeline and the SG3015/SG3018 guardrails.
- `[SerializableProperty]` guidance narrowed to its remaining purpose: custom getters and setter semantics the hooks cannot express.
- `[AnchoredDateTime]` documented alongside `[DeltaDateTime]` (now marked legacy, with the version-bump warning for converting between them).

**Files:** `dev-docs/serialization.md`, `dev-docs/timers.md`, `dev-docs/claude-skills/modernuo-serialization.md`, `dev-docs/claude-skills/modernuo-timers.md`, `dev-docs/runuo-migration-docs/02-serialization.md`, `dev-docs/runuo-migration-docs/03-timers.md`, and a condensed v4 addition to CLAUDE.md rule 9.

**Example refresh:** the skill's `BagOfSending` "custom properties" example was itself converted in #2587 — it is now quoted in its real post-conversion form as the canonical hooks example; the real-examples list points at `BaseWeapon.cs` for custom getters and `BaseLight.cs` for the drifting-timer + `MigrateFrom` pattern.

Verified by grep: zero references to the removed v3 attribute names remain anywhere in `dev-docs/` or `CLAUDE.md`.

CLAUDE.md

@@ -18,7 +18,7 @@ Apply these when writing or reviewing `.cs` files under `Projects/`.
 6. **Cancel timers in `OnDelete()`/`OnAfterDelete()`** — call `_token.Cancel()` or `_timer?.Stop()`
 7. **`STArrayPool<T>.Shared`** not `ArrayPool<T>.Shared` — single-threaded optimized, no locks
 8. **`PooledRefList<T>`** not `new List<T>()` on hot paths — zero GC pressure, stack-allocated ref struct
-9. **Serialization** — class must be `partial`, constructor needs `[Constructible]`, `TimerExecutionToken` must NOT have `[SerializableField]`. New classes: use `[SerializationGenerator(version)]` (omit `encoded`). When bumping versions, add `MigrateFrom(VXContent)` (X = previous version). Never modify `Deserialize(reader, version)` for version bumps — that method is only for pre-codegen legacy saves. When migrating from pre-codegen Serialize/Deserialize: pass `false` if old code used `reader.ReadInt()`, bump version +1, and keep old logic as `private void Deserialize(IGenericReader reader, int version)` → `dev-docs/runuo-migration-docs/02-serialization.md`
+9. **Serialization** — class must be `partial`, constructor needs `[Constructible]`, `TimerExecutionToken` must NOT have `[SerializableField]`. New classes: use `[SerializationGenerator(version)]` (omit `encoded`). Setters that coerce/veto/run side effects: use `[SerializableField]` args `allowFieldChange: nameof(BoolRefMethod)` / `fieldChanged: nameof(OldNewMethod)` — reserve `[SerializableProperty]` for custom getters. Serializable `Timer` members declare `[DeserializeTimer(nameof(Method))]` on the field (anchored by default — downtime preserves remaining delay; `wallClock: true` = absolute; method runs only when a timer was running at save). Conditional writes: `[SaveFlag(nameof(Should), nameof(Default))]` on the field. When bumping versions, add `MigrateFrom(VXContent)` (X = previous version). Never modify `Deserialize(reader, version)` for version bumps — that method is only for pre-codegen legacy saves. When migrating from pre-codegen Serialize/Deserialize: pass `false` if old code used `reader.ReadInt()`, bump version +1, and keep old logic as `private void Deserialize(IGenericReader reader, int version)` → `dev-docs/serialization.md`, `dev-docs/runuo-migration-docs/02-serialization.md`
 10. **No `Task.Run`/`new Thread()` for game logic** (tandem with rule #3) — game logic is the single-threaded event loop. Backgrounding is allowed only for work that does not itself touch game state (external service calls, large-file parse). **Prove the need before adding a thread**: measure **on-loop** time, not wall-clock (frozen world is the cost, player latency is not), and gate on `Environment.ProcessorCount` — off-loading creates no CPU and buys nothing on 1–2 cores. New workers go in the vetted table in `dev-docs/threading-model.md` with their measurement. When such work must *feed* game logic: run the heavy/I/O part off-loop and `ConfigureAwait(false)` its awaits so a continuation never resumes on the loop and silently foregrounds heavy work; then hand the result back **explicitly** — publish an immutable snapshot swapped via a `volatile` reference (the loop reads it lock-free), or marshal the apply step with `Core.LoopContext.Post(() => …)`, re-validating in the continuation whatever may have changed while it ran. Never touch game state off-thread; never let the scheduler decide where the heavy work runs → `dev-docs/threading-model.md`
 11. **Never assume era** — if code uses `Core.AOS`/`Core.SE`/etc., ask which expansion to target
 12. **Naming** — `_camelCase` private fields, `PascalCase` properties/methods/classes; don't flag legacy `m_` but use `_` for new code