Align core callable signatures and argument contracts - #9538

Scope

Follow up 15 concrete signature mismatches in the existing whats_left CI report against CPython 3.15.0. This is not a claim that the full signature inventory is fixed.

• Preserve omitted-vs-explicit-None behavior while supplying CPython Clinic display defaults for __lazy_import__ and _warnings.warn.
• Align AST, profiler callback, and JSON encoder parameter names.
• Give JSON scanstring its actual positional/default metadata and use truth conversion for strict (the prior Rust bool converter only accepted integers).
• Accept the Clinic-supported keywords for the two sys lazy-import setters.
• Require exactly three positional arguments to lock/RLock __exit__ before releasing, rather than accepting arbitrary varargs.
• Include the already reviewed --signature output option on the existing CI comparison command; it only prints details the script already computes, without adding scans/imports.

Two pure Rust tests check the actual warning/lazy-import parameter metadata. No Lib tests, assertions, snippets, dependencies or generic binder ordering are changed.

Validation

• Independent source review against pinned CPython d220409a13b77de7a6c30abe16ea90db01528edc: passed.
• Normal repository commit hooks and diff whitespace check: passed.
• Local Cargo compilation/tests and interpreter execution: not run; local dependency-cache/network restoration remains blocked. CI validation is pending for this exact head.
• The 15-item reduction is a source-based expectation, not yet an observed after-report. Existing full suites and the detailed comparator will measure it.

Remaining differences include warning stacklevel's i32 width vs Py_ssize_t and generic invalid-call error priority. This patch does not establish complete runtime or error-message parity.