feature: add RFC 8259 JSON parser/serializer and string utility extensions

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)

mkdocs.yml

@@ -136,6 +136,7 @@ nav:
         - MD5: src/core/enc/core.enc.md5.md
         - SHA-1: src/core/enc/core.enc.sha1.md
       - Formats:
+        - JSON: src/core/fmt/core.fmt.json.md
         - Targa (TGA): src/core/fmt/core.fmt.targa.md
       - Graphics:
         - Color: src/core/gfx/core.gfx.color.md