fix(dv): reject trailing bitmap data

[fallintoplace]

Summary

Reject trailing bytes after a portable roaring position bitmap.

Problem

The portable format starts with the number of keyed bitmaps, so that count frames the complete payload. The decoder reads exactly that many entries but does not verify that the input is exhausted afterward. As a result, an otherwise valid bitmap with arbitrary trailing bytes is accepted and the extra data is silently ignored.

For deletion vectors this weakens corruption detection because malformed inner bitmap data can pass decoding even when the outer DV length and CRC are valid.

Fix

After decoding the declared bitmap entries, require the input reader to have no bytes remaining.

Tests

Added framing coverage for empty and multi-key non-empty bitmaps. An end-to-end DV envelope test also appends bytes to a valid bitmap, rebuilds the matching length and CRC, and verifies the contextual decode error.

go test ./table/dv -count=1