Expose a public unit-to-frame accessor for raid frames

[dfrisone]

Adds `EllesmereUI.RaidFrames_GetFrameForUnit(unit)`, a small public accessor
that returns the actual button for a given unit token on our own raid
frames.

Context: EllesmereUIRaidFrames keeps its unit->button map (`unitToButton`,
`ns._partyUnitToButton`) private -- every existing reader is an internal
event handler in this same file. A companion addon (NaowhUI_SmartReminders,
part of the same suite) wants to highlight a specific raider's raid-frame
button for a new "raid-frame glow" reminder type, and needs a way to find
that button without reaching into this file's own locals.

The new function is a one-line combined lookup, the same shape every
internal reader in this file already uses (`unitToButton[unit] or
ns._partyUnitToButton[unit]`). No existing behavior changes -- purely
additive, zero cost unless called.