knowledge(performance): grouped query (Count + ColumnFilter = HAVING) for distinct values and duplicates

Adds use-grouped-query-for-distinct-values-and-duplicates with good/bad
samples, a worklist cue in al-performance-review, and registration in the
performance review-fixtures override.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

microsoft/skills/review/al-performance-review.md

@@ -47,6 +47,7 @@ Apply these targeted cues even when simple token overlap would rank the article
 
 - Worklist `design-covering-keys-from-read-pattern.md` for a changed secondary key alongside a filtered reader of its table, and `review-overlapping-keys-before-adding-an-index.md` when added or changed keys have overlapping leading fields. A changed key alone is not a finding; read and write workloads determine whether either rule applies.
 - Worklist `preserve-buffered-inserts-by-separating-target-reads.md` when a loop calls `Insert` and interleaves operations on the insert target or `Commit`. Worklist `aggregate-before-persisting-intermediate-results.md` when repeated grouping calculations and persistent intermediate summaries appear in the same processing path. Do not infer either pattern from `Insert` or `CalcSums` alone.
+- Worklist `use-grouped-query-for-distinct-values-and-duplicates.md` when a record loop sets a filter on a second record variable of the same table to the current row's value and calls `Count`, `IsEmpty`, or `FindFirst` only to decide whether that value is duplicated. Do not worklist it for a single uniqueness check on one record (for example in `OnValidate` or before `Insert`), an outer loop bounded to one parent document, a lookup against a different subset than the looped rows, a loop that acts on each row it finds, or a filtered/counted record that is temporary.
 - Worklist `cache-repeated-filtered-results-with-explicit-scope.md` only when the code or workload establishes repeated **complete** lookup keys (for example, querying the same filtered set in multiple passes, or measured key reuse), or shows a cache that omits result-affecting inputs. A single loop over possibly distinct keys does not establish reuse or justify a cache finding. Worklist `avoid-repeating-unchanged-validation.md` for repeated `Validate` of the same field in one path; do not worklist it from a single validation call.
 - Worklist `use-setautocalcfields-for-per-row-flowfields.md` when a record loop calls `CalcFields`, or when every row reads the same FlowField for a comparison, branch, or per-record action. Also worklist `calcsums-instead-of-calcfields-in-loop.md` when the loop accumulates one set total: use `CalcSums` directly only for stored source fields, never directly on FlowFields; a FlowField total requires deriving equivalent source filters from its `CalcFormula`.
 - Worklist `hidden-flowfields-still-calculate-before-bc26-opt-in.md` when a page control directly sources a FlowField and sets `Visible = false` or a visibility expression. Suppress it when the target is known to have BC26's **Calculate only visible FlowFields** feature enabled, or when the FlowField is cheap and intentionally preloaded.