Validate config JSON against schema

[Symmetricity]

This PR was prepared with AI assistance.

Tilemaker now validates config JSON against a schema before reading required
settings from it.

The schema lives in resources/config-schema.json so it can be read, reviewed,
and reused by editors or external validation tools. The build embeds that schema
into tilemaker:

• CMake reads resources/config-schema.json and generates config_schema.h in
  the build directory.
• The Makefile generates src/config_schema.h from the same schema file.
• The generated Makefile header is ignored and removed by make clean.

This avoids a runtime dependency on finding a schema file while still keeping
the schema separate from the C++ code.

Why

Invalid config files could previously fail in two unhelpful ways:

• Syntax errors only reported Invalid JSON file.
• Structurally invalid but syntactically valid configs could reach
  Config::readConfig, where unchecked RapidJSON access could assert.

This specifically improves the cases reported in #867 and #875. For example,
the config attached to #875 is now rejected before readConfig with:

Invalid JSON file: missing required fields "compress", "name", "version", "description" at #/settings.

Other examples now include:

Invalid JSON file: Missing a comma or '}' after an object member. at offset 41.

Invalid JSON file: invalid type at #/layers/water/minzoom: expected "integer", got string.

Testing

• make test_config_validator
• cmake -S . -B ... -DCMAKE_BUILD_TYPE=RelWithDebInfo
• cmake --build ... --parallel 8
• smoke-tested malformed JSON, schema-invalid JSON, and the config from Tilemaker v3 kept crashing on rapidjson::GenericValue<Encoding, Allocator>::operator[] #875
• make test
• git diff --check