compute: shift the index accounting object ids for the merged builtin set

The merge took main's `s<N>` object ids without this branch's own shift, leaving
`mz_catalog_server_index_accounting.slt` describing neither side.

Derived rather than guessed. Comparing this branch before the merge against the
pre-merge base shows its delta is +1 on every one of the 81 objects in the file,
with none unshifted, so the new builtin log precedes all of them. Main's delta
varies by position, +0 at id 543, +1 at 546, +3 at 839, which is three builtins
inserted at ascending positions and confirms the ids are contiguous and
position-derived rather than individually assigned. Composing the two is therefore
+1 on every object id in the merged file, including the one main added, since that
also sits after this branch's insertion point.

Checked against four anchors spanning main's three insertion points: `mz_comments`
543 to 544, `mz_metric_sinks` 545 to 546, `mz_materialized_views` 547 to 548, and
`mz_notices` 842 to 843.

The substitution matches `[sNNN␠AS␠` so it cannot touch the `[s2]` in
`IN␠CLUSTER␠[s2]`, which is a cluster id. Confusing the two is what made an
earlier attempt at this analysis wrong.

Part of CPU-226

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018ZVCMBSLdxzGus78ZKWhZz

Author: claude