feat: read appended data sections

[Michael-F-Bryan]

ULog Reader currently refuses files with DATA_APPENDED, which prevents buttctl logs inspect from reading PX4 logs containing appended hardfault data. The captured bench log ends part-way through a D payload, then starts a complete M hardfault message at the advertised offset, so merely ignoring the incompatibility flag would consume the appended message header as missing payload bytes.

This makes the streaming reader track its absolute byte position and treat each advertised offset as a section boundary. A header or payload interrupted by that boundary is discarded only up to the offset, then parsing resumes with the existing formats and subscriptions. Appended offsets are validated as present and strictly increasing; the public io.Reader API remains unchanged and does not require seeking.

With this branch, the original 17.8 MB bench log decodes through buttctl logs inspect as 290,037 records and includes the appended hardfault_plain multi-information group.