chore(data-structures): low-priority follow-ups from a data-structure review #1683

Findings from a review of the data structures, checked against dev. The two that pay off are in #1682 (Redis folder key count, groupings number parsing). What is left is small; each item says whether it is worth doing.

1. PoolEntryLock._lock is never active (worth a test first)

lib/core/database/connection_pool_lock.dart: createIfAbsent sets _lock, runs a block with no await, and clears it in finally in the same turn, so the while (_lock != null) await _lock loop never waits. De-duplication really comes from _pending[key]. The one thing the lock changes is re-entrancy: a call to createIfAbsent made from inside create() waits instead of running at once. The class has no tests and three pools use it (PostgreSQL, MySQL, SQLite).

• Add tests: two callers, one key, one create(); two keys in parallel; a failing create() clears the entry; create() that throws synchronously.
• Then drop _lock (or keep it and say why in the doc comment).

2. BooleanCompactColumn (probably not worth it)

Boolean columns are stored as 0/1 in Int64List (8 bytes per row, about 790 KB per column at 100k rows more than a bit array). A packed column touches formatting, sort, type detection and export. Only do this if a profile shows boolean columns dominating the heap.

3. StringInternPool does not evict

When maxEntries (4096) is reached, new strings are simply not interned. That is the right trade for a bounded pool; an LRU would add bookkeeping to a hot path. Only revisit if results with more than 4096 distinct short values are common.

Considered and rejected

• MetricHistory.record uses removeRange(0, 1) on a list of at most 120 samples every 5 s: a copy of 120 references. A ring buffer would be more code for no visible gain.
• GenericCompactColumn.formatter and isolates: noted on perf(results): carry the columnar dataset to sort / filter / export instead of List<List<String>> (memory 68 -> 17 MiB at 50k x 20) #1549, where the isolate hand-off is in scope.