fix TdtRawIO multi-block SEV file lookup (wrong path + case mismatch)

[ree-gupta]

Fixes multi-block SEV file lookup in TdtRawIO. Two bugs caused silent fallback to TEV, resulting in all channels reading identical data:

1. path variable at L290 was stale (from iterdir() loop at L103 or reassigned to tank root at L170), never pointed to the block subdirectory where SEV files live
2. Filename used lowercase ch but TDT writes Ch, not found on case-sensitive filesystems (Linux)

Fix: use self.dirname / segment_name for the correct block directory, and [cC]h glob to match both cases (same pattern the single-block path already uses).

Not caught by tests because aep_05 (the only multi-block test dataset) has no SEV files.

Closes #1814