feat: save/restore full interpreter snapshots

[dylan-sutton-chavez]

Summary

Adds full interpreter snapshots: freeze a paused run (waiting on receive(), sleep(), frame(), or a host call) into a portable blob and resume it later, even on a fresh page load.

• save_state / restore_state capture and revive the whole VM. Val bits and heap slots are kept verbatim, so references, cycles, and interning survive.
• The blob embeds the source plus a bytecode fingerprint, so restore rejects a different program or compiler build.
• state_globals / state_stack introspect a parked run without resuming it.
• Pause and snapshot programs with no suspension point
• Docs and tests.

Host-side resources (DOM handles, sockets, in-flight host calls) are not part of a snapshot.

issue: #28
cc: @vrthra