Function materialize 

pub fn materialize(
    base_toml: &str,
    overlay: &BTreeMap<String, String>,
    seed: u64,
    run_root: &str,
    timestamp: &str,
    slug: &str,
) -> Result<String>

Builds one trial’s RunConfig TOML.

Starts from base_toml (the text of oniro_cli default-config, or a user-supplied base), overrides each overlay entry at its dotted key with a value coerced to the base’s type there, and sets the trial’s seed and run directory (run.root/run.timestamp/run.slug). The timestamp and slug together must make root/<timestamp>-<slug> unique across concurrent trials and a trial’s replicates (the caller derives them from the trial number and the replicate seed).

Errors

SpikeError::Materialize if the base config is not a TOML table, if a dotted key is empty or traverses a non-table, if a value cannot be coerced to the base’s type at its key, or if the merged config cannot be re-serialized.