Adds getTotalDocCount() to FulltextIndexWriter (default -1 = not tracked)
and wires it through all writer implementations:

- DefaultIndexWriter: commit() then numDocs() before close() — no extra I/O
  since close() calls commit() internally; accurate after pending deletes.
- MultiplexingIndexWriter: sums sub-writer counts (filters negatives for
  mounts that were never opened).
- PooledLuceneIndexWriter: delegates to the wrapped writer.
- ElasticIndexWriter: LongAdder initialised to prevTotal (incremental) or 0
  (reindex); incremented/decremented in afterBulk() and deleteDocuments().
- LocalIndexWriter (NRT): returns -1 — NRT docs are in-memory only; the
  persistent count is updated by the next async cycle.

closeWriter() reads getTotalDocCount() and persists it to
:status/totalIndexedNodes when the value is >= 0. For the Elastic gap
(empty reindex returns indexUpdated=false so the legacy :status block never
ran), a separate additive block writes totalIndexedNodes and
REINDEX_COMPLETION_TIMESTAMP for the !indexUpdated && reindex case.

Kill switch FT_OAK-12247 (AtomicBoolean FT_OAK_12247_DISABLE, default false
= tracking active) is registered as a FeatureToggle in
LuceneIndexProviderService following the FT_OAK_12193 precedent.

IndexStatsCollector and IndexStatsImpl are removed (net ~200 lines).