feat: retro Blaster Master HUD MVP (#2)

[lemillermicrosoft]

Closes #2.

Requested by deehoc via Discord DM (as the initial !new-addon blaster_master concept — assistant defined the MVP):

create a new addon inspired by Blaster Master for NES. You define the mvp. Get creative!!

What lands

• Retro HUD frame (draggable): ARMOR (HP, green→yellow→red) + AMMO (resource, tinted by power type) + PILOT label. Position persists.
• BOSS ROOM alert banner on targeting elite/rare/rareelite/worldboss, with per-GUID 10s cooldown.
• 8-bit sound cues (built-in WoW sounds as chiptune stand-ins) on enter-combat, low-HP threshold crossing, kill (PARTY_KILL), level-up. Gated by muted.
• /blaster / /bm slash: toggle, mute, reset, hp <0..1>.
• SavedVariables: enabled, muted, lowHpThreshold, point.

Not in this PR (post-MVP)

Custom chiptune audio, config UI, per-class palettes, vehicle-mode takeover, LibSharedMedia/LibDBIcon.

Ship with !ship blaster_master when you've smoke-tested it in-game.