fix(cdm): apply Custom Icon to aura-tracked custom buffs

[wanghws]

What does this PR do?

A Custom Spell ID on a Buffs bar offers Add Custom Icon in its right-click menu, but
the icon never changed: the buff kept showing its own art (screenshots below).

Cause. A custom spell ID on a buffs-family bar has no stored duration, so it is not a CDM
frame at all -- it renders in the engine aura container hung off the bar's end (_AC in
EllesmereUICdmHooks.lua). ApplyCustomIcon only stamps claimed CDM frames and our own injected
frames, so it never reaches these buttons. #1425 recorded the underlying limit: one engine group
renders every custom aura on the bar, and while auras are secret Lua cannot tell its buttons
apart, so per-spell settings have nowhere to land.

Fix. A spell that has a Custom Icon gets a single-spell group of its own in the same
container. Every button in that group is then known to hold that spell without reading the
aura, so the group's style can carry the art:

• The style hides the engine-stamped icon (the same SetShown + alpha pair Only Show Numbers
  already uses) and shows a texture of our own in its place. The texture is created in the
  button's creation window (_AC.InitExtra, the only place a region may be parented to an engine
  aura button) and anchored to the icon, so shape masks, zoom crop, Cropped and Only Show Numbers
  all follow it.
• Which spells carry a Custom Icon is part of the container's structural signature; the fileID
  is not. Re-picking an icon restyles that group in place (_AC.SyncFixedIcons) instead of
  rebuilding, because a rebuild abandons the old container and its engine-created group buttons
  can never be freed (AK.ReleaseContainer). Styles of groups a rebuild drops are removed from
  AK.styles.
• The Custom Icon popup's confirm now also runs UpdateCustomBuffAuraTracking: the reanchor it
  already queued never reaches the container.

Notes:

• Nothing changes for a bar with no Custom Icon on an aura-tracked custom: it keeps the single
  shared group and the exact signature it had, and the per-spell resolve is gated on
  ns._cdmAnyCustomIcon.
• A split-out spell flows after the bar's other custom auras (the engine lays groups out in
  order).
• A split does not grow the per-build button count: AuraKit notes group pools are sized at 10x
  their visible count, and the split moves one visible slot from the shared group to the new one.
• The other per-spell rows in this menu (Buff Glow, Border, Reverse Swipe, ...) still do not
  reach these icons and are out of scope here; the audio cues are covered separately by CDM: play buff gain/loss audio cues for aura-tracked custom buffs #1712.

How was it tested?

Tested in-game on live against this checklist:

• Custom Spell ID on a Buffs bar, Add Custom Icon 461860: the aura renders with the custom art
  while active.
• In combat and in M+ (auras secret): the custom art holds through aura refreshes and stack
  changes.
• Bar icon shape, Cropped and Only Show Numbers: the custom art follows each.
• Re-picking a different icon two or three times in a row: each applies immediately.
• Clearing the Custom Icon: the aura reverts to its own art.

Screenshots

Before -- Custom Icon 461860 saved for the spell, the buff still shows its own art:

After -- the buff shows the custom art:

Checklist

• New settings default OFF -- N/A: no new setting; an existing per-spell setting now takes
  effect where it was silently ignored
• Zero cost while disabled: the per-spell resolve runs only once ns._cdmAnyCustomIcon is
  set, and bars without a Custom Icon on an aura-tracked custom keep the exact container and
  signature
• Cheap while enabled: no events, polling or timers added; allocation only when the container
  is (re)built or restyled, never per aura event or per frame
• No writes onto Blizzard-owned frames: only regions on AuraKit engine buttons we build, in
  their creation window; no hooks added
• Tested in-game on live; no version gates or pre-Midnight APIs added

https://claude.ai/code/session_01NYsCUA5DSWPU1wHWFZv2p7