Head-amp VALUE + name/icon overrides accepted when a physical input is FIRST registered
(mirrors makeSource's init). Ignored when a channel joins an input another channel
already sources — a split inherits the shared preamp, it never resets it. Trim/polarity are
NOT here (2026-07-30) — they are channel-owned, never part of the shared physical record.
Head-amp VALUE + name/icon overrides accepted when a physical input is FIRST registered (mirrors makeSource's init). Ignored when a channel joins an input another channel already sources — a split inherits the shared preamp, it never resets it. Trim/polarity are NOT here (2026-07-30) — they are channel-owned, never part of the shared physical record.