feat: add runtime engine for query execution

[zharinov]

Summary

Add a runtime VM engine that executes compiled bytecode against tree-sitter ASTs and produces JSON output.

Components

Engine core (engine/):

• vm.rs - Virtual machine with backtracking support
• cursor.rs - TreeCursor wrapper with Plotnik navigation semantics
• checkpoint.rs - Checkpoints for backtracking
• frame.rs - Call frame arena for recursion
• effect.rs - Runtime effects (Node, Arr, Obj, etc.)
• materializer.rs - Effect stream → JSON value conversion
• value.rs - JSON value types and serialization
• trace.rs - Execution trace for debugging
• error.rs - Runtime error types

Bug fixes (compile/):

• Fix scalar array captures producing nulls (missing Node+Push effects)
• Fix recursive patterns flattening captures (missing Obj/EndObj scopes)

Tests

23 snapshot-based tests covering captures, quantifiers, alternations, recursion, anchors, fields, and regression cases.

Test plan

• All 704 tests pass
• Clippy passes