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fix(spatial): never serve an empty index (caused mass postcode mis-mapping)

The locations index could load 0 rows yet report ready=true: startupLoad
adopted any on-disk index that opened cleanly without checking it had rows,
and rebuild swapped in a freshly-built index even when its load returned
nothing. An empty index answers KNN with absurd far-away 'nearest' results,
so the nightly postcode->area remap wrote areas tens of km away into the DB
(e.g. BB4 6RE -> 'River Ribble' 35km north), corrupting thousands of live
post subjects.

- rebuild(): refuse to replace the live index with a 0-row build; keep the
  previous good index and surface the failure.
- startupLoad(): if an existing on-disk index opens but has 0 rows, discard
  and rebuild instead of adopting it.
- regression tests for both guards.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XdgqWQHJzEebnFr2oi8aXK

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