Add PTP hardware timing support to MTL and FFmpeg TX paths

[sunilnom]

• Wire MTL_FLAG_PTP_ENABLE/PI/UNICAST_ADDR into both the direct MTL TX path (src/mtl/mtl_tx.c) and the FFmpeg mtl_st20p muxer path (src/ffmpeg/ffmpeg_tx.c), with a sync-notify callback logging master_utc_offset/delta.
• app_context.h: add ptp_enable/ptp_pi/ptp_unicast fields.
• config_reader.c/.h: hardcode PTP defaults (enable=true, pi=true, unicast=false) instead of reading them from JSON.

Verified on hardware (Intel I225-V/igc) with both enable_mtl_tx=true and enable_mtl_tx=false builds: PTP enables cleanly (tx pacing ptp, ptp_init pi controller) with no grandmaster present on the test network (sync cnt 0, as expected).

Description

Checklist

Code Quality

• Code follows project style guidelines
• No unnecessary debug logs or commented-out code
• No hardcoded values / secrets

Testing

• Unit test added/modified accordingly
• Perform manual basic sanity testing at system level

Review Readiness

• PR title and description are clear and meaningful
• Story/Task IDs are linked

Documentation

• README or relevant docs updated (if applicable)

Security

• No sensitive data exposed (keys, passwords, tokens)
• Input validation added where needed

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Documentation content changes
• Testing
• Other... Please describe: