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`.

dev-docs/runuo-migration-docs/03-timers.md

@@ -305,7 +305,7 @@ In RunUO, timers are commonly started in `Deserialize()`. In ModernUO, use `[Aft
 `_token.Cancel()` can be called on a default token, a stopped token, or an already-cancelled token. No null checks needed.
 
 ### 4. Timer.DelayCall Still Exists
-`Timer.DelayCall()` is still available and returns a `Timer` object. Use it when you need the `Timer` reference (e.g., for `[DeserializeTimerField]`) or state-carrying overloads.
+`Timer.DelayCall()` is still available and returns a `Timer` object. Use it when you need the `Timer` reference (e.g., for a serialized timer field with `[DeserializeTimer]`) or state-carrying overloads.
 
 ### 5. Custom Timer Classes Are Still Possible
 For complex timer logic (e.g., `Corpse.DecayTimer`), you can still subclass `Timer` with `OnTick()`. But prefer the fire-and-forget pattern for simple cases.