Audio and video streams from C64 Ultimate hardware always use the same
format (PAL or NTSC). Audio sample rates are format-specific, derived
from the color subcarrier frequencies:

- PAL:  47,983 Hz (from 4433618.75 Hz color subcarrier)
- NTSC: 47,940 Hz (from 3579545.45 Hz color subcarrier)

This results in different audio packet intervals:
- PAL:  192 samples ÷ 47983 Hz = 4,001,417 ns/packet
- NTSC: 192 samples ÷ 47940 Hz = 4,005,006 ns/packet

Implementation:
1. Added C64_PAL_AUDIO_SAMPLE_RATE and C64_NTSC_AUDIO_SAMPLE_RATE constants
2. Audio sample rate set automatically when video format is detected
3. Audio packet interval calculated dynamically from sample rate
4. No independent audio format detection needed

Changes:
- src/c64-protocol.h: Define audio sample rate constants
- src/c64-types.h: Add audio_sample_rate field to context
- src/c64-audio.c: Calculate audio_interval_ns dynamically
- src/c64-source.c: Initialize with PAL default, set from format hint
- src/c64-video.c: Set audio_sample_rate when format detected
- doc/c64-stream-spec.md: Correct audio packet interval calculations

Remove hardcoded magic number 4001417ULL and calculate interval
consistently using (192 * 1e9) / sample_rate formula.

…mport

Skip directories silently instead of logging warnings when
paths ending with separators are passed to palette parser.
Only log warnings for actual VPL files that fail to open.

…mport

Use proper filesystem APIs (stat/GetFileAttributes) to detect
directories instead of just checking for trailing slashes.
Only log warnings for actual VPL files that fail to open.

Replace direct POSIX/Windows filesystem calls with OBS wrappers:
- os_stat() instead of stat() (c64-file.c, c64-palette.c)
- os_opendir/os_readdir/os_closedir instead of opendir/readdir/closedir
- Removed Windows-specific #ifdef blocks for directory enumeration
- Unified path handling using OBS directory APIs

This eliminates platform-specific code and ensures consistent
behavior across Windows, Linux, and macOS.