feat(core): implement physical types with NewType pattern

Story 1.2: Physical Types (NewType Pattern)

- Add Pressure, Temperature, Enthalpy, MassFlow types
- Implement SI base units with conversion methods
- Add arithmetic operations (Add, Sub, Mul, Div)
- Add Display and Debug traits
- Comprehensive unit tests (37 tests)
- Add PSI and Fahrenheit conversions
- Code review fixes applied

All tests passing, clippy clean

Cargo.toml

@@ -1,7 +1,7 @@
 [workspace]
 members = [
     "crates/components",
-    # "crates/core",      # Will be added in future stories
+    "crates/core",
     # "crates/solver",    # Will be added in future stories
 ]
 resolver = "2"