feat: move internals.h, exception_translation.h, and pybind11_fail out of line

The internals accessor family (get_internals, ensure_internals, the
local-internals key and capsules, exception translators) moves into
internals-inl.h; per-module identity is unchanged because the
function-local statics move with their functions into whatever binary
each module links. Also adds common-inl.h (pybind11_fail) and
exception_translation-inl.h. Tiny hot accessors and all templates stay
in the headers.

Assisted-by: ClaudeCode:claude-fable-5