TypedPython: depth precheck, fast entry and inlining for acyclic callees (spectral-norm) #147

Approved by the user (2026-10-04) from docs/investigations/typedpython-vs-cython-numba-lpython.md. spectral-norm is 6.3x (M2 needs 10x, #41); its hot path is a leaf call per inner iteration paying tp_enter_call/tp_leave_call.

Design (agreed in #41): a callee outside every call cycle adds at most k frames (k static). A precheck depth + k <= limit at the cycle entry or wrapper selects a fast entry without enter/leave, so the C compiler can inline it. If the precheck fails, the call runs the normal per-call path, never an early RecursionError.

Done when: tests compare with the interpreter at every depth between limit-k and limit (result, RecursionError, and at which call); a mutation that takes the fast path despite a failed precheck fails them; spectral-norm re-measured (interleaved, quiet window).