feat(export): add JSON fallback for snapshot export (#974)

[Taofik01]

Closes #974

What this PR does

Adds a JSON export option for StateSnapshot so users can export a specific
state frame to a human-readable file for manual auditing.

Changes

• simulator/src/export.rs — new file with ExportFormat toggle (Json/Binary),
  export_snapshot(), to_json_pretty(), to_binary(), and load_snapshot()
• simulator/src/lib.rs — registered pub mod export

How it works

• ExportFormat::Json → pretty-printed JSON matching the Soroban RPC schema
• ExportFormat::Binary → compact bincode encoding (existing internal format)
• ExportFormat::from_flag("json") parses the new CLI internal flag
• load_snapshot() auto-detects format from the file bytes

Acceptance criteria met

• JSON export is human-readable and matches the Soroban RPC schema
• CLI can output JSON snapshots via ExportFormat::from_flag()
• Pretty-printing enabled via serde_json::to_string_pretty

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