Coastal Steel is an original 96-second, 140 BPM FM action score with six sections: approach, breakwater riff, offshore pursuit, stormwall bridge, citadel assault and the return to the coast. Fingered bass, muted power chords, an overdriven guitar melody, a contrasting lead and restrained strings sit over kick, snare, hats and occasional tom fills. The closing phrase resolves to the opening E-minor harmony for continuous mission playback.

The package is independently toggleable, uses mod API 1 and has no mod dependencies. Iron Coast missions depend on it and reference iron_coast_music:iron_coast through music_track. It uses the existing Rust MUS/GENMIDI FM music bus, so the game's music volume and mute controls apply. The original game instrument bank is required locally; it is not copied here.

The score, deterministic composition recipe and provenance are included in Git. No original-game melody, recording or instrument data is bundled. Regenerate the exact score with:

python3 mods/iron_coast_music/build_music.py

The engine services MUS events at 70 Hz. This score uses five melodic channels and standard percussion on channel 15, releases notes explicitly, and ends after 6,720 ticks. It stays within the checked package music limits. The looping flag is declared in content.json; neither a Rhai script nor an ordinary sound-effect command is needed for playback.

Validation uses the actual Rust decoder and locally installed original FM instrument bank. A 100-second stereo render is non-silent through the first loop, has no clipped samples, and mutes to exact digital silence. The recipe also checks paired note events and the 6,720-tick duration. Numeric results are in validation.json; human listening and normal-play sound balance remain part of wider campaign testing. The locally rendered preview remains outside Git because it uses the original instrument bank.