Arena: stop early GetContentType calls from destroying the reload hint

[Krathe82]

Problem

The existing ARENA RELOAD FIX (trust the saved lastContentType hint while IsInInstance() recovers after a reload) was being defeated by its own neighbours, leaving the arena header hidden after a reload in arena — worst case on a combat reload mid-round, where the frames could stay hidden until the round ended.

Sequence:

1. On a reload, FinalizeHeaderInit → UpdateHeaderVisibility calls GetContentType() at ADDON_LOADED — before PLAYER_ENTERING_WORLD arms useContentTypeFallback.
2. If WoW's APIs aren't ready yet, that early call either nils DandersFramesCharDB.lastContentType (not-in-raid branch) or overwrites it with "openWorld" (in-raid + instanceType "none" — arena groups are raid groups, and IsInRaid() can recover before IsInInstance()).
3. By the time PEW arms the fallback, the hint it relies on is gone.

Out of combat, the ArenaDetectionRetry / ARENA_PREP backups recover. Mid-combat they can't (secure Show is blocked), so the one window that can fix it — the combat-safe ADDON_LOADED window the addon already uses for combat reloads — ran with a wrong content type.

Fix

• Snapshot the hint once at load (DF._contentTypeHintAtLoad, captured in Core's ADDON_LOADED handler plus a self-capture in GetContentType for ordering safety) and read the fallback from the snapshot, so later live-field writes can't destroy it.
• Arm the fallback at ADDON_LOADED on a /reload (player already exists at ADDON_LOADED only on a reload — same detection the header init uses), so the combat-safe FinalizeHeaderInit window already resolves "arena" and can show the arena header. Fresh logins still never arm it: a stale "arena" hint from a previous session would misclassify the login zone, and the fallback only clears once real detection returns a definite answer.
• Extend the fallback to the IsInRaid()==true + instanceType "none" case, which previously classified an arena reload as "openWorld" and showed raid frames over the arena header.

No behaviour change outside the reload-recovery window: the first confident IsInInstance()/roster read clears the fallback and resumes normal detection, exactly as before.

Testing

• Structural syntax check passes; fallback/branch logic traced against all four entry orderings (ADDON_LOADED vs PEW, in/out of combat).
• /reload in arena prep and mid-round: header visibility + sorting recover via the armed fallback instead of waiting for ArenaDetectionRetry.