feat: add query compilation to bytecode

[zharinov]

Summary

• Add Thompson NFA-style query compilation (compile.rs)
• Rename emit.rs → codegen.rs and enhance with linked/unlinked paths
• Add StringTableBuilder for bytecode string interning
• Add dump tests for bytecode output verification
• Update Query API with emit() returning bytecode

Key components:

• Compiler - Thompson construction from query AST to IR
• codegen::emit() - Unlinked bytecode (StringId references)
• codegen::emit_linked() - Linked bytecode (NodeTypeId/NodeFieldId)
• Query::emit() / LinkedQuery::emit() - Public API

Depends on: #186-#191 (all previous extraction PRs)

Test plan

• cargo test -p plotnik-lib compile passes (6 tests)
• cargo test -p plotnik-lib codegen passes (15 tests)
• cargo test -p plotnik-lib dump passes (6 tests)