kernel/api: introduce kernel API specification framework

Add a framework for formally documenting kernel APIs with inline
specifications. This framework provides:

- Structured API documentation with parameter specifications, return
  values, error conditions, and execution context requirements
- Runtime validation capabilities for debugging
  (CONFIG_KAPI_RUNTIME_CHECKS)
- Support for both internal kernel APIs and system calls

The framework stores specifications in a dedicated ELF section and
provides infrastructure for:
- Runtime querying of API documentation
- Integration with existing SYSCALL_DEFINE macros

A KUnit suite (CONFIG_KAPI_KUNIT_TEST) covers registration, lookup,
parameter and return value validation, and JSON export.

No specifications are added here; subsequent patches add them for
individual syscalls. With CONFIG_KAPI_SPEC=n the SYSCALL_DEFINEx()
expansion is unchanged. CONFIG_KAPI_RUNTIME_CHECKS is a debug option:
a syscall whose arguments violate its specification returns -EINVAL
without running.

Assisted-by: LLM
Signed-off-by: Sasha Levin <sashal@kernel.org>

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