Custom topic injection mode with per-topic intervals

[bracyw]

Changes

Added --topic "TOPIC=INTERVAL_MS" flag to the simulator for selecting specific topics with custom per-topic loop intervals. This is the "injection mode" from #289 — only the specified topics are published, each on their own frequency.

Also gated imd_poll behind cfg(target_os = "linux") to fix macOS builds (matches the existing linux-only socketcan dep in Cargo.toml).

Notes

Refactored simulate_out so component creation and filtering now happen in main at startup. This means invalid topic names fail immediately with a clear error instead of silently inside the async task. Extracted compile_regex_patterns helper to deduplicate the blacklist/whitelist regex loops while I was in there.

The --topic flag conflicts with --enable-topic and --disable-topic since they're different modes.

Test Cases

• simulate --topic "BMS/Pack/Voltage=30000" — only publishes BMS/Pack/Voltage at 30s intervals
• simulate --topic "BMS/Pack/Voltage=30000" --topic "BMS/Pack/Current=10000" — publishes both at their own intervals
• simulate --topic "Nonexistent/Topic=1000" — fails at startup with "Unknown topic(s): Nonexistent/Topic"
• simulate --topic "BadFormat" — fails with "expected TOPIC=INTERVAL_MS"
• simulate --topic "BMS/Pack/Voltage=0" — fails with "Interval must be positive"
• simulate (no flags) — all topics published at default intervals, unchanged behavior

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Closes #289