feat: add repl preload state and checkpoint flags

[rlippmann]

What changed

• Added explicit preload flags for REPL/CLI:
  • --initial-state-json
  • --initial-state-file
  • --initial-checkpoint-json
  • --initial-checkpoint-file
• Implemented strict mutual exclusion between state preload and checkpoint preload options.
• State preload uses engine.import_json(...); checkpoint preload uses engine.import_checkpoint_json(...).
• Added UTF-8 file loading for preload file flags.
• Added fail-fast preload validation before processing stdin.
• Kept default REPL behavior unchanged when no preload flags are provided.
• Added/updated tests for preload success paths, mutual exclusion, invalid payloads, JSON-mode compatibility, and preload file I/O failures.
• Added brief README and CLI help coverage clarifying authoritative state preload vs checkpoint continuation preload.
• Added CI contract coverage job for controller module (100% gate), aligned with existing REPL and engine coverage jobs.

Why this was needed

• Supports explicit host-controlled startup state for scripting and REPL workflows.
• Keeps authoritative state transport and runtime continuation restoration as separate concepts.
• Preserves deterministic behavior and clear boundary semantics for 0.7.

Closes #138