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)

src/asuro.pas

@@ -117,7 +117,8 @@ uses
     wasm.test, 
     wasm.test.framework,
     driver.video.windows,
-    driver.bus.usb.xhci;
+    driver.bus.usb.xhci,
+    core.fmt.json;
  
 procedure kmain(mbinfo: Pmultiboot_info_t; mbmagic: uint32); stdcall;
  
@@ -352,9 +353,11 @@ begin
      core.ds.prio.UnitTest;
      driver.storage.vfs.UnitTest;
      driver.storage.test.UnitTest;
+     boot.splash.update(95, 'Running test suite...');
      core.enc.fnv1a.UnitTest;
      core.enc.djb2.UnitTest;
      core.ds.bloom.UnitTest;
+     core.fmt.json.UnitTest;
 
      { Initialize driver.video.desktop environment }
      boot.splash.update(100, 'Booting desktop...');