feat: event-driven target acquisition with a reaction-time gradient (#2601)

Fixes walk-up aggro latency (up to a full 10 s of obliviousness) and hardens the reacquire gate so no state can silence acquisition, while turning `AcquireOnApproach` into the reaction-time knob for future per-creature intelligence tuning.

### Why

`AcquireFocusMob` re-armed the 10 s `ReacquireDelay` **before** scanning, success or failure. A creature that scanned an empty room was blind for 10 s to a player walking up — walk-up aggro latency was uniform in 0..10 s. Waking from sector sleep stacked the AI timer's 0–3 s construction stagger on top. And `NextReacquireTime` is not serialized: on hosts whose tick counter starts negative (GCP pass-through), the 0 default blocked **all** acquisition shard-wide after a restart until the counter crossed zero.

### What

**Event-driven reaction — `AcquireOnApproachDelay` (the intelligence gradient)**
- The paragon `AcquireOnApproach` bool becomes a `TimeSpan` on every creature: an enemy moving inside `AcquireOnApproachRange` (10 for all creatures — on-screen reactive aggro; the periodic scan keeps the wide `RangePerception` sweep) *clamps* the next scan to at most the delay. Repeated steps cannot shorten it further — one scan per delay period, not per step or think.
- `Zero` (paragons) also prods the AI timer: the ranked scan engages within a wheel turn — the old snap, minus the special-cased engage path. The target now comes from the normal FightMode ranking instead of whichever mobile happened to move, and the `Combatant == null` guard stops re-engage spam.
- The 2 s default reads as "took a beat to notice you"; larger values are dumber; `ReacquireDelay` alone is the oblivious floor. Mover checks are the approach logic's `IsEnemy` + `CanBeHarmful` (so pets count and hidden movers are excluded via `CanSee`), with `IsEnemy` first to cheaply reject same-team wild creatures wandering past. The check rides the `OnMovement` callback every step already pays for — no polling added.

**Gate correctness**
- Every scan re-arms the full `ReacquireDelay`, success or failure (classic semantics; reaction time is the approach path, not the poll).
- Self-healing by construction: a deadline further out than `ReacquireDelay` is an illegal state and reads as open — no wedged or wrapped value can silence acquisition beyond one delay period.
- `NextReacquireTime` is seeded from a live tick on deserialize (the GCP negative-tick blackout).

**AI timer wake**
- Activation (sector wake, spawn, resurrection) starts within a 0–256 ms spread instead of the 0–3 s construction stagger, which read as lag.
- The stagger's real job — keeping same-speed cohorts out of lock-step (the RunUO town artifact) — is now a zero-mean ±period/8 jitter on each **idle** think, so phases random-walk apart within seconds and can never re-lock. Instrumentation showed why a one-shot spread can't do this job: the timer wheel fires within ±1 ms, so with 10 creatures on a 500 ms period some pair collides on nearly the same phase ~75% of the time (birthday paradox) and then steps in the same loop iteration *forever*. Jitter is scoped to passive speed: engaged cadence stays exact, since pursuit timing anchors to real step times.

**Debug**
- The `AcquireFocusMob` scan message no longer re-arms the shared 5 s debug cooldown, which swallowed every AI's "I have detected X" transition line.

**API change** for custom scripts: `AcquireOnApproach` (bool) → `AcquireOnApproachDelay` (TimeSpan). Documented in `content-patterns.md` § Target Acquisition, `runuo-migration-docs/09` + `11`, and the migration skill checklist.

### Tests

`AcquisitionTests`: both scan outcomes honor `ReacquireDelay`; a 60 s-wedged gate still acquires; enemy movement clamps the deadline (same-team wild movers and out-of-range movers ignored); repeated movement cannot shorten below the delay; `Zero` opens the gate and prods without a direct engage. Full suite: 755 UOContent green.

dev-docs/claude-skills/migrate-from-runuo/migrate-items-mobiles.md

@@ -30,6 +30,7 @@ description: >
 - `Name = "text"` -> `public override string DefaultName => "text";`
 - Expression-bodied overrides: `public override int Meat { get { return 1; } }` -> `public override int Meat => 1;`
 - AI movement calls lose the `run` flag: `MoveTo(m, true, range)` -> `MoveTo(m, range)` (also `WalkMobileRange`, `ApproachTarget`, `MoveToPoint`, `PathFollower.Follow`); the Running bit is derived from step pace -> `dev-docs/runuo-migration-docs/09-items-mobiles-creatures.md` § AI Movement
+- `AcquireOnApproach` (bool) -> `AcquireOnApproachDelay` (TimeSpan; `Zero` = old instant behavior) -> same doc § Target Acquisition
 
 ## Anti-Patterns
 - Using `_field--` instead of `Property--` (bypasses MarkDirty tracking)

dev-docs/claude-skills/modernuo-content-patterns.md

@@ -29,7 +29,9 @@ description: >
    overridden). Prefer `npc-speeds.json` buckets (`SpeedClass`); `SetSpeed()` sets think
    AND clears move overrides, `SetMoveSpeed()` sets move only. The client `Running` bit is
    derived from the step pace (`BaseAI.ShouldRun`); movement APIs take no run argument --
-   see `dev-docs/content-patterns.md` § Creature Speeds
+   see `dev-docs/content-patterns.md` § Creature Speeds. Reaction time to approaching
+   enemies is `AcquireOnApproachDelay` (TimeSpan gradient; `Zero` = paragon snap, 2s
+   default, `ReacquireDelay`-only = oblivious) -- see § Target Acquisition
 8. **`OnThink` overrides must be excess-call tolerant** -- it fires more often than the
    think cadence (player commands prod it; speed-ups reschedule it). Gate consequential
    work on a tick-count deadline (subtraction form) or make it idempotent; bare per-call