feat: implement JSON stdlib module with consent-aware parsing

Implemented complete JSON module for WokeLang standard library.

Features:
- JSON parsing (parse) with proper type conversion
- JSON stringify with escape handling
- Consent-based capability checking (std.json.parse, std.json.stringify)
- Support for all JSON types: null, bool, number, string, array, object
- Escape sequence handling (\n, \t, \", \\, etc.)
- Nested structure parsing (arrays and objects)
- Permission denial for functions/channels/results (non-serializable)

Type Mapping:
- null → Unit
- boolean → Bool
- number → Int/Float
- string → String
- array → Array(Vec<Value>)
- object → Record(HashMap<String, Value>)

Security:
- Uses Capability::Custom for permission checks
- Integrates with CapabilityRegistry.request()
- Permissive mode for testing

Test Coverage:
- 8 JSON-specific tests (parse null, bool, number, string, array, object, stringify, permissions)
- All tests passing (95/95 total)

Files:
- Created: src/stdlib/json.rs (426 lines)
- Modified: src/stdlib/mod.rs (enabled json module, registered functions)

Progress Update:
- Stdlib: 75% → 80% complete
- Overall project: 70% complete (maintained)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: Test