Add core.fmt.json — a complete RFC 8259 JSON library with recursive-descent parser, serializer, builders, dot/bracket path accessors, and full cleanup. Supports all 6 JSON value types plus JSON_ERROR for parse error reporting. Numbers stored as Double via x87 FPU. Unicode \uXXXX escapes handled (ASCII range emitted, non-ASCII replaced with '?'). Supporting changes: - core.strings: add int64ToString, doubleToStr (scientific notation for very large/small values, fixed-point for normal range, trailing zero trimming), plus unit tests for both - asuro.pas: wire core.fmt.json.UnitTest into the boot test suite - mkdocs.yml: add JSON nav entry under Formats - doc/src/core/fmt/core.fmt.json.md: full API documentation - doc/src/core/core.strings.md: document int64ToString and doubleToStr - doc/planning/json.md: implementation plan and design spec - doc/planning/webgpu.md: WebGPU-over-WASM design plan (software rasterizer backend for WASM apps importing webgpu.h)