Clarify accepted output metric null semantics

[malon64]

Summary

• make accepted-output report metrics preserve null for unknown values and exact 0 for computed zeroes
• apply the shared contract across Parquet, Delta, and Iceberg metric mapping paths
• add focused docs and tests for serialization and sink-specific metric branches

Validation

• cargo fmt
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test -p floe-core --test unit unit::io::write::metrics:: -- --nocapture
• cargo test -p floe-core --test unit unit::report::accepted_output:: -- --nocapture
• cargo test -p floe-core --test unit unit::io::write::delta_write::delta_commit_metrics_from_log_bytes_ -- --nocapture
• cargo test -p floe-core --test unit unit::io::write::iceberg_write::write_iceberg_table_ -- --nocapture
• cargo test -p floe-core --test unit unit::run::report:: -- --nocapture
• cargo test -p floe-core --test integration local_run_parquet_metrics_track_chunked_output_files -- --nocapture
• cargo test -p floe-core --test integration local_delta_run_respects_partition_by_and_reports_metrics -- --nocapture
• cargo test -p floe-core --test integration local_run_writes_iceberg_table_and_report_fields -- --nocapture
• cargo test -p floe-cli --test output -- --nocapture

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