tests: pre-flight CIRCUITPY free-space check in audioio runner

STM32F4 CIRCUITPY is only ~2 MiB. With a leftover code.py + stale
WAVs the runner can't fit the 5 test WAVs (~1.3 MiB), and macOS
surfaces the resulting ENOSPC as a misleading "Operation not
permitted" — the first attempt to run this suite on a fresh dev
machine wasted real time chasing it as a permission issue. Sum the
bytes we're about to write up front and bail with a concrete
"need / free / short" summary so the next contributor knows
immediately to clear unrelated files.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>