feat: embed TOML data files in crate via include_str!()

[gerchowl]

Summary

The TOML material data files (~50 KB total) should be embedded directly in the crate so MaterialDb works without an external data directory.

Problem

Currently MaterialDb::open(data_dir) requires a path to TOML files, and from_pymat_data() only works within the py-mat monorepo (uses CARGO_MANIFEST_DIR to find ../src/pymat/data/). When used as a crates.io dependency, there's no built-in way to load materials.

Proposal

Embed all 7 category TOML files via include_str!():

const METALS_TOML: &str = include_str!("../data/metals.toml");
const SCINTILLATORS_TOML: &str = include_str!("../data/scintillators.toml");
const PLASTICS_TOML: &str = include_str!("../data/plastics.toml");
const CERAMICS_TOML: &str = include_str!("../data/ceramics.toml");
const ELECTRONICS_TOML: &str = include_str!("../data/electronics.toml");
const LIQUIDS_TOML: &str = include_str!("../data/liquids.toml");
const GASES_TOML: &str = include_str!("../data/gases.toml");

impl MaterialDb {
    /// Load the built-in material database (no external files needed).
    pub fn builtin() -> Self { ... }
}

Justification

• Total data size is ~50 KB — negligible impact on binary size
• The material database IS the crate's core value — shipping without it defeats the purpose
• Every downstream user (e.g. strata) would otherwise need to vendor or symlink the TOML files
• MaterialDb::builtin() becomes the zero-config default; open(path) remains for custom/extended databases

Context

Used by strata (Rust MC transport engine) as the material definition layer alongside nucl-parquet for cross-section data.