[ColinLeeo]

Summary

Repeated batch queries whose tag filter selects exactly one device rebuilt the device and timeseries-index metadata on every call. query_table now resolves the requested field columns once through a bounded LRU and executes them as prepared queries: one column through query_prepared, several aligned columns through query_prepared_multi. Queries that select a TAG column, use row-by-row mode, or whose filter matches zero or several devices keep the previous path.

• TsFileReader::prepare_table_series() resolves (table, column, tag filter) to a PreparedLocator through the existing device index and parses it once.
• The prepared query paths accept a TsBlock row capacity (block_rows) instead of always allocating PREPARED_BATCH_ROWS, so cached queries keep the caller's batch_size block boundaries. The existing C entry points keep their signatures; tsfile_reader_query_prepared_ex and tsfile_reader_query_prepared_multi_ex carry the new parameter.
• Python exposes prepare_table_series() and caches single-device field queries behind TSFILE_QUERY_CACHE_SIZE (default 1024, 0 disables). close() releases the cached handles.
• Only "not cacheable" error codes fall back to the regular path. I/O, short-read, corruption and OOM errors still reach the caller, so the existing read-failure propagation tests pass unchanged.

Measurements

323 devices x 20,000 points, 2,000 point queries per run, warm page cache, alternating cache on/off:

query	cache off (median)	cache on (median)
1 field column	229 us	54 us
2 field columns	287 us	102 us

The means include one miss per device; steady state is roughly 50 us (one column) and 100 us (two columns) per query.

Scope

• The cache applies only when the tag filter selects exactly one device and every requested column is a stored field. TAG columns, row mode and multi-device filters keep the previous code path; non-cacheable results are remembered so they are not retried on every call.
• Results are unchanged: the new tests compare cached and uncached output across one and two columns, column order, time ranges containing nulls, compound filters, eviction, TAG-column fallback and row mode. tests/test_reader_sources.py still verifies that every injected read failure propagates.
• block_rows also removes the hard-coded 65,536-row block that prepared queries used when no row limit was given ([Perf][C++] Point queries rebuild index metadata on every call; prepared series queries are ~7x faster but need fixes #1080).

Tests

• python/tests/test_query_table_cache.py: 17 cases.
• Full Python suite: 620 passed, 1 skipped.
• C++ library and Cython extensions built locally; clang-format applied to the touched files.

Closes #1081. Relates to #1080.