Seed performance knowledge corpus (22 articles + AL samples)

Converts an existing performance-review prompt into 22 atomic
knowledge articles under microsoft/knowledge/performance/, each
paired with AL samples under samples/performance/<slug>/ demonstrating
the anti-pattern and/or the best practice. The full set seeds the
corpus the microsoft/skills/al-performance-review leaf skill matches
against and validates the READ knowledge-file format end-to-end.

Every article conforms to the READ contract: six required frontmatter
fields, Description always present, no fenced code blocks, sample
code referenced by repo-relative path. Each article is marked with a
blockquote 'Seed article' note so domain stewards can extend or
restructure them freely.

Articles (ordered by concern area):

Database query efficiency
- use-findset-with-next (AA0181)
- avoid-findfirst-with-next (AA0233)
- only-fetch-records-you-use (AA0175)
- use-findset-readonly-by-default
- use-setloadfields-for-partial-records
- use-addloadfields-in-report-layouts
- use-calcsums-to-aggregate-filtered-sets (file: use-calcsums-for-flowfield-totals.md)
- avoid-calcfields-in-loops
- add-sift-keys-for-flowfields (AA0232)
- use-isempty-for-existence-checks

Filter and key optimization
- filter-before-find
- set-current-key-to-match-filters

Temporary tables and transactions
- use-temporary-tables-for-intermediate-data
- keep-transaction-scope-short
- avoid-user-interaction-in-transactions
- avoid-commit-inside-loops

Record operations
- prefer-get-for-primary-key-lookups
- use-insert-false-when-skipping-triggers
- prefer-direct-record-over-recordref

Strings, codeunits, events
- use-strsubstno-for-message-formatting
- use-single-instance-codeunits-for-caching
- keep-event-subscribers-lightweight

samples/README.md documents the sample-folder convention and makes
clear the samples are demonstration-only, not derived from BC base
application source, with unique object IDs in the 50100-50199 range.

Rubber-duck pass caught: a misleading good.al in avoid-calcfields-in-loops
(fixed by switching to a hoistable CalcFields scenario), an invalid
event subscriber signature in keep-event-subscribers-lightweight
(fixed by adding var xRec), normative guidance leaked into the
Description of use-findset-readonly-by-default (moved to Anti Pattern),
a missing sample pair for keep-transaction-scope-short (added), and
muddy FlowField/CalcSums framing (retitled and clarified).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

microsoft/knowledge/performance/use-single-instance-codeunits-for-caching.md

@@ -0,0 +1,27 @@
+---
+bc-version: [26..28]
+domain: performance
+keywords: [singleinstance, cache, codeunit, session]
+technologies: [al]
+countries: [w1]
+application-area: [all]
+---
+
+# Use SingleInstance codeunits for session caching
+
+> **Seed article.** Converted from an existing performance-review prompt to bootstrap the BCQuality performance corpus. Domain stewards should expand, restructure, and refine as needed.
+
+## Description
+
+A SingleInstance codeunit lives once per session. Variables on it survive across calls, which makes it the natural home for data that is expensive to compute, read often, and stable for the duration of the session — feature flags, configuration snapshots, setup records. Each cached value avoids a SQL read per subsequent call site.
+
+## Best Practice
+
+Store long-lived, read-often, rarely-changing data on a SingleInstance codeunit, populated lazily on first access. Keep the cached footprint small: a handful of booleans, a setup record, a few derived values. Be explicit about invalidation if the source can change during the session.
+
+See sample: `samples/performance/use-single-instance-codeunits-for-caching/good.al`.
+
+## Anti Pattern
+
+Reading the same setup record on every call from every caller, instead of caching it, repeats a SQL round-trip that has no business happening more than once per session.
+