fix(sensing): fuse only coherent frame cohorts

[ruvnet]

Summary

• Select one timestamp domain for each sensing cycle.
• Prune active frames to the freshest cohort within the configured hard fusion guard.
• Preserve single-node processing and engine-error audit accounting.
• Add regression coverage for mixed ESP32-S3/C6 frame widths, frame rates, and custom guards.

Validation

• cargo test --workspace --no-default-features
• cargo build --release -p wifi-densepose-sensing-server --bin sensing-server --no-default-features
• Local four-node hardware burn-in observation: 607 seconds, 4/4 active nodes, zero governed fusion errors, and 60 witness changes.
• No firmware changes; the existing node firmware was used.

Restricted/demoted output can still occur when the soft privacy guard closes. That is intentional policy behavior rather than a fusion error.