Circular buffer write race condition

It looks like there is a problem in `circ_buf_write()` in how it doesn't wrap the whole call in interrupt masking and instead relies on `circ_buf_push()` protecting each byte insert, with only one check for MIN() and no for loop exit on overflow. In practise this may be mitigated by having everything serialised on the main thread.

`circ_buf_write()` is also very inefficient by pushing a single byte at a time.

Start by locating circ_buf_write() and circ_buf_push() in the firmware source. Trace how interrupt masking, MIN(), and overflow handling interact during writes, then examine whether the byte-at-a-time behavior contributes to the reported inefficiency. Done means the race and overflow behavior are addressed without relying on main-thread serialization, with relevant firmware tests or validation run if available.

Written by the indexing model from the issue text.