Arena: don't hide late-loading teammates (incomplete-nameList fallback)

[Krathe82]

Live + alpha reports: in non-rated arenas, zoning in early and having the last teammate join a bit later leaves their frame missing for the whole round (joining last yourself is always fine); often described as "first round of every arena, 1-2 players missing", and a mid-match /reload can leave the sort wrong afterwards.

Root cause: with sorting on and Self Position First/Last (the default), the arena header runs in NAMELIST mode. A teammate can exist before their name resolves (UnitName returns nil/UNKNOWNOBJECT while they load in — exactly the trickle-in pattern of non-rated queues). A nameList built in that window omits or mis-names them, so the secure header filters their frame out — and nothing repairs it: name resolution fires no GROUP_ROSTER_UPDATE, and once the gates open, combat blocks header attribute writes for the rest of the round. Joining last is fine because everyone else's names resolved long ago; solo shuffle is unaffected because FrameSort owns arena sorting when active.

Fix:

• BuildArenaNameList now reports completeness; an unresolved member is omitted rather than added as a literal "Unknown" (which could never match their real name).
• On an incomplete build, ApplyArenaHeaderSorting shows everyone in INDEX order instead of applying a filtering nameList — unsorted beats missing — and a capped 0.5s retry loop re-sorts once names resolve. UNIT_NAME_UPDATE nudges the retry too (gated on the incomplete flag), with the timer as primary since that event is unreliable.
• ApplyArenaHeaderSorting's combat early-return now queues pendingSortingUpdate instead of silently dropping — an arena round's combat lasts minutes, so e.g. a post-/reload apply mid-round previously vanished, leaving the header unsorted (the "sort is screwed after reloading" half of the report).

Worst case under the fix is a few seconds of unsorted-but-complete frames during prep, then correct sorting.

[Krathe82]

Follow-up commit from a deep audit of the arena population/sorting pipeline against Blizzard's SecureGroupHeaders.lua:

• Roster-sourced nameLists: for raid-kind units (arena = raid1-5), the secure header matches nameList tokens against GetRaidRosterInfo names, not UnitName — and the roster usually knows a late-joiner's name before their player object loads. BuildArenaNameList now builds from the roster, so in the common late-join case the list is complete and matchable immediately: frames appear sorted right away, with the INDEX fallback from the first commit kept as the safety net for true roster gaps.
• FrameSort integration: SortArenaFrames had the same late-join hole this PR fixes natively (an unloaded teammate's name was skipped or inserted as a literal "Unknown" token — frame filtered out for the whole round). It now uses roster names, falls back to INDEX on an incomplete roster, and hooks into the shared retry/nudge machinery via the new DF:SetArenaSortIncomplete. The retry is FrameSort-aware: it re-requests FrameSort's order instead of calling ApplyArenaHeaderSorting (which would just yield and clear to INDEX).
• A UNIT_NAME_UPDATE nudge now re-arms the retry safety cap, so a player loading slower than the ~12s window still gets re-sorted.