[Json] Implement sce::Json::Value and String (construct / set / destroy) (#169)

* [Json] Implement sce::Json::Value and Json::String construct/set/destroy

libSceJson previously only had the Initializer/MemAllocator setup path.
The Value and String classes themselves were entirely absent, so a
Prospero title that builds a JSON tree (Quake PPSA01880 does, to shape
a web-API request) hit unresolved imports and faulted on the call. The
imports it left unresolved right before its access violation are exactly
these Value ctors/setters and String ctor/dtor.

Model the Value/String payload host-side (JsonObjectHeap), keyed by the
guest `this` pointer, following the handle-shadow pattern already used
by Ngs2Exports. The guest object bytes are deliberately not written:
these objects are usually stack-allocated with an unknown real layout,
and writing a guessed layout risks smashing an adjacent stack canary
(the same hazard the AudioOut2 context-param note in this tree records).
Constructors and setters follow the Itanium ABI and return `this` in rax,
which is correct whether the real setter returns void or Value&.

Covered NIDs (complete-object C1/D1 variants, matching the observed
imports): Value(default/bool/long/ulong/double/ValueType/char*/String),
Value::~Value, Value::set(bool/long/ulong/double/ValueType/char*/String),
Value::clear, String(char*/default/copy), String::~String.

Only the payload the guest can reach through library methods is modelled;
direct guest reads of the object bytes are out of scope and would need
observed layout evidence.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* [Tests] Add SharpEmu.Libs.Tests covering the Json Value/String exports

First test project for SharpEmu.Libs (xunit), the SharpEmu.Libs.Tests
layout the maintainer already agreed to in issue #36.

- A FakeCpuMemory (single contiguous region) drives the exports at the
  CpuContext level with no live guest.
- Direct-call tests: ctor/setter round-trips for bool/int/uint/double
  (read from xmm0)/char*/String/ValueType, destructor cleanup, and the
  graceful-degradation paths (missing String shadow and a faulting char*
  pointer both fall back to the empty string instead of throwing).
- Registration test: a real ModuleManager scans SharpEmu.Libs and the
  nine NIDs Quake left unresolved now resolve to the libSceJson exports
  and dispatch cleanly (returns `this` in rax).

InternalsVisibleTo exposes JsonObjectHeap to the test assembly. The test
project's packages.lock.json is committed for CI locked-mode restore;
CI does not run tests yet, left as a maintainer decision.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* [Json] Add Initializer::setGlobalNullAccessCallback

Quake calls it during kexPSNWebAPI::Initialize and treats the
not-found error as fatal for the whole Np Web API bring-up. Store the
guest hook (never invoked by this HLE: shadows degrade to defaults
instead of dereferencing missing members) and return success.

Verified against the dump: the "setGlobalNullAccessCallback failed
(0x80020002)" line is gone and kexPSNWebAPI::Initialize now logs
"Np Web API Initialized"; the next blockers are sceNpAuthCreateRequest
and sceUserServiceInitialize ordering, outside libSceJson.

Also pins both Json test classes to one xunit collection: they share
JsonObjectHeap statics and parallel class execution raced ResetForTests
against a running test.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

tests/SharpEmu.Libs.Tests/SharpEmu.Libs.Tests.csproj

@@ -0,0 +1,22 @@
+<!--
+Copyright (C) 2026 SharpEmu Emulator Project
+SPDX-License-Identifier: GPL-2.0-or-later
+-->
+
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <IsPackable>false</IsPackable>
+    <GenerateDocumentationFile>false</GenerateDocumentationFile>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\SharpEmu.Libs\SharpEmu.Libs.csproj" />
+    <ProjectReference Include="..\..\src\SharpEmu.HLE\SharpEmu.HLE.csproj" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" />
+    <PackageReference Include="xunit" />
+    <PackageReference Include="xunit.runner.visualstudio" />
+  </ItemGroup>
+</Project>