Refactor (4/5): factor shared code out of hook.c/frame.c

[yuuu]

Stacked on #42.

• set_session / disarm replace reset code that was repeated in hook_s_install, hook_s_uninstall and mrb_mrdebug_gem_final. disarm still leaves in_callback alone, since uninstall can run mid-callback via MRDebug.detach. gem_final now also releases the session and the cached filename string; before, they stayed GC-registered.
• invoke_on_line takes forced directly, so the invoke_on_line_forced wrapper is gone. report_position does the filename lookup and filename_value call that three call sites used to repeat.
• Removed the mrb_nil_p(hook.deferred_bp) && !hook.in_callback guard before check_method_call. It was always true at that point: on_line never sets deferred_bp, and run_in_dbg_context has already cleared in_callback.
• frame.c: paused_frame() does the paused-context + frame_at lookup that frame_position and frame_binding both repeated.

Tested:

• rake test:unit: OK 3015, KO 0
• rake test:smoke and rake picoruby:smoke: 0 failures each
• Manual run of line + method breakpoints (b Foo#bar, b Array#sort, p, bt): same output as before

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https://claude.ai/code/session_01CpfQQRdc3R13RafigfCNVk