[core][cpp] Fix mob-linking behavior impacting Einherjar

[MarkWaldron]

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What does this pull request do?

Resolves an issue with mobs of different families linking in Einherjar. It's a combination of two issues, one being MOBTYPE_BATTLEFIELD, and the other being force-link.

What this would look like is: pulling an Utgarth Bat would link the chamber's boss, even though the boss has no family or sublink relationship to bats.

Root cause

Regression in bbc703d620 ("add ability for mob families that don't link to link with subfamilies"). That commit reorganized the inner match in FindPartyForMob and added a force-link clause OR'd alongside the family/sublink clauses, which let force-link mobs with sublinks bridge two otherwise-disjoint groups.

Fix

In FindPartyForMob, treat force-link and family/sublink as mutually exclusive matching paths:

• If the mob force-links, it only joins other force-link mobs.
• Otherwise it joins via family or sublink as before.

Contained to one function in src/map/zone_entities.cpp. ShouldForceLink(), mob pools, family mods, and zone settings are unchanged.

Retail behavior reference

The June 9, 2008 FFXI version update explicitly changed Einherjar linking behavior:

"Monsters within Einherjar chambers will now link with one another in the same manner as enemies in other general areas."

Source: https://www.bg-wiki.com/ffxi/Version_Update_(06/09/2008)

Original-release Einherjar (June 6, 2007) had wave-wide linking regardless of proximity; SE patched that out in 2008. Current retail Einherjar links along normal family rules, which is what this fix restores. Boss-add interactions (Vampyr_Jarl transformation, Motsognir demon adds, Hildesvini Djiggas, etc.) are scripted spawn mechanics rather than links, and are unaffected by this change.

Test plan

Restart the map server before each test (party assignment runs at zone load).

• Einherjar Wing 3, Vampyr Jarl chamber. Pull a single bat with a ranged attack. Verify only bats engage and Vampyr Jarl stays idle.
• Einherjar Wing 3, non-Vampyr_Jarl chamber (e.g. Freke). Same check.
• Einherjar Wing 1 and Wing 2 chambers. Same check.
• Engage an Einherjar boss directly. Confirm normal-family adds and chamber boss adds (Vampyr_Bats, Vampyr_Wolf for Vampyr_Jarl) still behave correctly.
• Dynamis. Pull one mob, verify Dynamis-style zone-wide linking still occurs.
• Limbus or Salvage run. Verify battlefield mobs still link inside the same battlefield instance and not across instances.
• Open-world zone with bats and vampyrs (e.g. Castle Zvahl Baileys). Verify they still link via their shared sublink.