app_mixmonitor: perform directional silence padding after both streams to avoid double-insert

[alonsoac]

Motivation

• Fix a critical logic bug where padding was applied inside each per-direction write loop, which could double-insert silence when fr_read and fr_write both contained frames in the same audiohook cycle and thus stretch recordings.
• Keep padding notifications low-noise (one-time per direction) and preserve the end-of-recording summary of total silence inserted.

Description

• Change mixmonitor_thread to write all read frames and all write frames first (updating counters only after successful ast_writestream calls), and perform at most one balancing pad operation afterwards based on the final counter delta.
• Add per-datastore counters and flags to struct mixmonitor_ds: read_samples_written, write_samples_written, read_silence_samples_inserted, write_silence_samples_inserted, read_silence_log_shown, and write_silence_log_shown to track totals and one-time logging.
• Introduce mixmonitor_write_silence() to emit zeroed frames in chunks and mixmonitor_samples_to_ms() to convert sample counts to milliseconds, and update end-of-recording logging to report totals.
• Ensure writes are skipped on ast_writestream failure and only successful writes advance sample counters.

Testing

• Attempted a local compile check with make -j2 apps/app_mixmonitor.o, which failed because the source tree requires ./configure to be run before make, so the build could not be completed in this environment.
• No other automated tests were executed in this environment.

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Codex Task