Add flippercheck: a validator for .sub / .ir / RTTTL / playlist files

[munzzyy]

This adds flippercheck, a small static checker for the file types this repo is full of - .sub, .ir, RTTTL music, and Sub-GHz playlists. It reads files and reports what would break on a real Flipper. It never transmits and doesn't need a Flipper plugged in. Pure Python 3, standard library only, one script.

Why: files rot quietly here - a key that's the wrong length, a RAW capture with a stray line in the middle, a frequency the CC1101 can't tune, a pause long enough to overflow the transmit timer - and none of it shows up until you try to send. There was an IR-only linter (darmiel/fff-ir-lint) but nothing checked .sub files, and it's been untouched since 2024. This covers all four in one tool.

The rules come from the Flipper firmware source and the CC1101 datasheet, then checked against the real files in this repo. Where the firmware tolerates something the docs forbid, it follows the firmware - e.g. RAW captures that start on a negative value are fine, so they aren't flagged. I ran it across every .sub file in the repo (11k+) to make sure it doesn't cry wolf on good files.

Running it here actually turned up a few genuinely broken files, e.g.:

• Sub-GHz/Sleep_Files/* encode a multi-hour pause as one RAW value (~-2,000,000,000 us), past the firmware's 30-bit transmit limit, so it truncates to a wrong delay - that matches the old "sleep files don't delay" reports.
• A couple of LRS pager files set to 467.75 MHz, above the CC1101's top band, so a real Flipper can't tune them.
• A few files with the spaces stripped out of the key so it no longer parses as bytes.

Usage and the full list of checks are in flippercheck/README.md. Tests: python3 flippercheck/tests/test_flippercheck.py. Happy to adjust placement or anything else.