Replace the bootstrap mixin-based compiler with a new IR-based pipeline:
Source → Lexer → Parser → Resolver → TypeChecker → IR Builder → Optimizer → Backend

New architecture (compiler/):
- semantics/: scope resolution, type system, type checker
- ir/: intermediate representation with stable node IDs
- optimizer/: DCE via call graph BFS, CSE, constant folding
- backend/bash/: bash code generation from IR
- backend/awk/: AWK code generation sharing same IR
- symbols/: symbol table with LSP-ready serialization

Key improvements:
- Explicit shell command detection (IRCall.is_shell_cmd)
- Namespace prefixing only in bash backend, not in resolver
- DCE reduced from 580-line fixed-point to ~40-line BFS
- CSE uses stable node_id instead of Python id()
- Shell commands in assignments use || true for set -e safety
- No local keyword in global scope (when/foreach)
- Coproc uses exec to prevent orphan child processes

All 400 tests pass. Verified with real-world unified-theme-switcher service.