enhancement: add Pattern.compile() and Pattern.compile_system() to OGS

[rororowyourboat]

Problem

All 4 non-OGS DSLs provide Model.compile() → GDSSpec and Model.compile_system() → SystemIR convenience methods. OGS Pattern has neither — users must call standalone functions:

# Other DSLs (consistent)
spec = model.compile()
system = model.compile_system()

# OGS (different)
from ogs import compile_to_ir, compile_pattern_to_spec
ir = compile_to_ir(pattern)
spec = compile_pattern_to_spec(pattern)
system = ir.to_system_ir()

OGS also has a different error hierarchy (DSLTypeError/DSLCompositionError vs ValidationError/CompilationError).

Proposal

Add convenience methods to Pattern:

• Pattern.compile() → GDSSpec (delegates to compile_pattern_to_spec)
• Pattern.compile_system() → SystemIR (delegates to compile_to_ir + to_system_ir)

This preserves the existing standalone functions while giving users a consistent entry point.

Context

The architectural difference (Pattern-centric vs Model-centric) is intentional. This is about API ergonomics, not changing the underlying design.