feat: add bonus experience game mechanic

[HarleyGilpin]

Description

Implements Bonus XP Weekend: while active (events.bonusExperience.enabled), players earn
boosted experience starting at 2.7x, decaying every 30 minutes spent online down to 1.1x.
Progress shows in the XP counter orb hover tooltip (multiplier, time elapsed, bonus xp earned).

Changes

Engine

• Experience.multiplier — per-player multiplier applied on top of world.experienceRate;
  not persisted, resets to 1.0 each login. Reusable by other xp-boosting content (auras, pendants).

BonusExperience.kt

• Login while active: enables tooltip (varbit 7232), starts a 1-minute timer, sends welcome message
• Timer increments time online (varbit 7233, persisted) and recalculates the multiplier — the decay
  table matches the client's own table in clientscript 338 exactly (verified by disassembling the
  cache), including the inclusive ≤30 minute first interval
• experience hook accrues the bonus portion into the bonus xp counter (varp 1878, x10 units,
  persisted) and reruns the orb refresh script (776)
• While disabled: persisted progress is reset on login

Data & config

• Varbit/varp definitions in gameframe.varbits.toml / gameframe.varps.toml
• refresh_bonus_experience (776) mapping in client.scripts.toml
• events.bonusExperience.enabled game property, default false

Tests

• BonusExperienceWorldTest — activation on login, 2.7x grant + bonus tracking, decay after
  31 minutes online, progress reset when disabled
• BonusExperienceTest — multiplier decay table boundaries
• ExperienceTableTest — engine multiplier applied with/without rate

Known limitation

The 634 client has no bonus-xp orb art (sprite 5568 absent), so the only display is the XP orb
hover tooltip. The tooltip box wraps the trailing "xp" of "Multiplier: 2.7xp" onto a second line —
client-side layout in clientscripts 776/569; fixing it needs a cache edit.

[GregHib]

Isn't it supposed to use this interface (548:41) or is that a different revision?

Edit: Looks like it was used but later removed and the cache doesn't have the xp+ orb icon.

[HarleyGilpin]

Isn't it supposed to use this interface (548:41) or is that a different revision?

Greg and I discussed via Discord that this interface wasn't introduced until the end of 2011, Client scripts were not available yet. We will need to update the cache to implement bonus xp orb and this interface in the future.