fix(encoder): allow encoding zero-length deletion files with HeaderFlagNoChecksum

[Aditya-9-6]

Summary

Fixes #103

Fixes a contradiction in Encoder.Close() that made it impossible to encode zero-length (deletion) LTX files when HeaderFlagNoChecksum was set.

Details

1. encoder.go:194 unconditionally required enc.trailer.PostApplyChecksum == ChecksumFlag whenever Commit == 0.
2. However, for files with HeaderFlagNoChecksum, Trailer.Validate() enforces that PostApplyChecksum == 0 (post-apply checksum not allowed if non-zero).
3. This PR updates encoder.go:194 to only require ChecksumFlag when checksums are enabled (!enc.header.NoChecksum()). When HeaderFlagNoChecksum is set, Trailer.Validate() already verifies that PostApplyChecksum == 0.
4. Adds unit tests in encoder_test.go verifying both valid zero-commit NoChecksum encoding/decoding and rejection of invalid non-zero checksums under NoChecksum.