fix(c++): auto-include Generators.hpp and use real type names in multi-source usage comments

[schani]

Bug

In C++ --source-style multi-source output, two related problems made generated code confusing or uncompilable without manual workarounds:

1. Missing Generators.hpp include in the umbrella header. Generators.hpp holds all the to_json/from_json overloads for the generated types, but the umbrella header (e.g. myapp.hpp, built by emitMultiSourceStructure) never included it. Any code that only includes the umbrella header and calls nlohmann::json::parse(...).get<SomeType>() fails to compile/link because the required to_json/from_json overloads aren't visible — users had to manually discover and add #include "Generators.hpp".
2. Misleading usage comments. Every generated file's leading "how to use this" comment used the current file's own name instead of a real top-level type name, e.g. myapp.hpp data = nlohmann::json::parse(jsonString); or helper.hpp data = nlohmann::json::parse(jsonString); — neither of which is valid or meaningful C++.

Root cause

Both bugs are in packages/quicktype-core/src/language/CPlusPlus/CPlusPlusRenderer.ts:

1. emitMultiSourceStructure calls this.startFile("Generators.hpp", true) but never adds "Generators.hpp" to this._generatedFiles. The umbrella header's #include list is built later by iterating _generatedFiles, so Generators.hpp was silently omitted.
2. startFile only used forEachTopLevel to emit one usage-comment line per real top-level type name when typeSourceStyle was true (single-source). For multi-source (typeSourceStyle false), it fell back to using the file's own basename instead.

Fix

• Add the missing this._generatedFiles.add("Generators.hpp") call so the umbrella header includes it.
• Always use forEachTopLevel to emit the usage-comment lines with real top-level type names, regardless of typeSourceStyle.

Test coverage

• test/unit/cplusplus-multi-source.test.ts (new): asserts the multi-source umbrella header contains #include "Generators.hpp", and that usage comments across generated files name the real top-level types (ChunkCache, BufferPath) rather than the generated filename.
• test/languages.ts / test/fixtures.ts: added a new cplusplus-multi-source fixture (CPlusPlusMultiSourceLanguage) that runs the existing C++ JSON fixture suite with source-style: multi-source against pokedex.json (multiple top-level types), so the generated multi-source output is actually compiled and round-tripped by the fixture harness.
• test/fixtures/cplusplus/main.cpp: changed to #include only the umbrella header (previously it manually included both the umbrella header and Generators.hpp, which masked bug WIP: Nested types #1 from ever being caught by the fixture suite).

Verification

• npm run build passes.
• npx vitest run test/unit — 165/165 unit tests pass, including the new cplusplus-multi-source.test.ts.
• QUICKTEST=true FIXTURE=cplusplus-multi-source script/test — passes locally (compiles with g++, runs, round-trips pokedex.json through the multi-source generated C++ code).
• Manually re-ran the exact repro from the issue/triage comment: the umbrella header now contains #include "Generators.hpp" and usage comments read // ChunkCache data = nlohmann::json::parse(jsonString); / // BufferPath data = nlohmann::json::parse(jsonString); instead of myapp.hpp data = ... / helper.hpp data = ....
• CI will additionally validate the full C++ fixture matrix (including other renderer-option combinations) across languages this PR doesn't specifically target.

Fixes #2438

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