Fix reflash callback ownership and error handling

[drebbe-intrepid]

Fixes #241.

set_reflash_callback() now owns its registered handler until replacement or unregistration and keeps an in-flight reference so callbacks can safely unregister themselves. Invalid handlers and failed symbol lookup preserve the existing registration. Progress callbacks release their return values and report exceptions through sys.unraisablehook; Unicode conversion also works on CPython 3.14. Existing callable, reflash_callback method, no-argument stdout, and None disable modes remain supported.

Validation: rebuilt the extension on Windows x64 with CPython 3.14.5 and 3.12.10; all 42 tests passed on each, including 12 new hardware-free native mock cases. The lifetime regression failed against the original build. Coverage includes dropped owner, replacement, unregistration, self-unregistration, invalid handlers, attribute errors, missing native symbol, result cleanup, exceptions, Unicode stdout, worker-thread delivery, and delivery during registration. No hardware operations performed.

Independent critic /root/critic explicitly agreed with the final diff and independently ran all 42 tests. Builder also explicitly agreed before publication. Reviewed commit: 7b74272a35bf5380fdf121583d472bd710032149 (identical reviewed blobs). Parent coordinator retains CI acceptance, integration, hardware testing, and merge responsibility.