feat: add terrain-core, tma, and unitdat crates with parsing functionality

- Introduced `terrain-core` crate for loading and processing terrain mesh data.
- Added `tma` crate for parsing mission files, including footer and object records.
- Created `unitdat` crate for reading unit data files with validation of structure.
- Implemented error handling and tests for all new crates.
- Documented object registry format and rendering pipeline in specifications.

crates/terrain-core/Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "terrain-core"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+nres = { path = "../nres" }
+
+[dev-dependencies]
+common = { path = "../common" }