Fix #if SQLITE_VEC_ENABLE_RESCORE guards wrapping non-rescore logic

Six sites used #if SQLITE_VEC_ENABLE_RESCORE to guard _vector_chunks
skip logic that applies to ALL non-flat index types. When RESCORE was
compiled out, DiskANN and IVF columns would incorrectly access flat
chunk tables. Two sites also missed DiskANN in the skip enumeration,
which would break mixed flat+DiskANN tables.

Fix: replace all six compile-time guards with unconditional runtime
`!= VEC0_INDEX_TYPE_FLAT` checks. Also move rescore_on_delete inside
the !vec0_all_columns_diskann guard to prevent use of uninitialized
chunk_id/chunk_offset, and initialize those variables to 0.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>