reader: Fix bits_read sync on all seek variants

[PedroDeSanti]

Closes #606.

Bug

Reader::seek updates bits_read only for some SeekFrom variants:

• SeekFrom::Start(n) skips the update when n == 0
• SeekFrom::Current(n) skips when n <= 0
• SeekFrom::End(_) never updates it

After one of those seeks, the inner reader sits at the right byte position but bits_read keeps the old value. The generated from_bytes body then computes idx = bits_read / 8 and slices input.get(idx..) at the wrong offset. The result is either:

• Err(Incomplete(NeedSize { bits: N })) even though the parse already succeeded, or
• Ok(...) with a wrong consumed-byte count.

Fix

Drop the SeekFrom match and read the position back from the inner reader directly:

let new_pos = self.inner.seek(pos)?;
self.bits_read = usize::try_from(new_pos)
    .unwrap_or(usize::MAX)
    .saturating_mul(8);
Ok(new_pos)

Covers Start(0), negative Current, and any End the same way. The saturating conversion keeps bits_read correct on targets where usize is narrower than u64 and for streams larger than usize::MAX / 8 bytes.

Tests

New regression tests in tests/test_seek.rs:

• test_seek_from_start_zero_rewind - round-trips a struct using seek_from_start = "0" after a forward seek (minimal repro of the bug).
• test_seek_from_end_rewind - round-trips a struct using seek_from_end, the variant that was never updating bits_read before the fix.
• test_seek_from_current_negative_rewind - round-trips a struct using negative seek_from_current (original scenario in Reader::seek doesn't rewind bits read, causing leftover slice error #606).
• test_seek_error_preserves_bits_read - directly asserts bits_read is unchanged when the inner seek returns an error.

Repro

I put together a small repo with three scenarios from #606 showing upstream vs patched outputs side by side:

https://github.com/PedroDeSanti/deku-seek-bug-tests

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[PedroDeSanti]

done

[PedroDeSanti]

done