fix: close four Copilot review findings left open by merged PRs

[jchable]

Six findings came out of the Copilot reviews on #78, #81 and the §5.1 documentation PR. All three PRs merged; four of the six findings were still live on dev.

Not fixed here, on purpose

• Utf8Offsets.ToUtf16 accepting a non-boundary offset — already closed by fix(producer): empty the findings register, then fix what an adversarial review found in that work #81, which replaced >= with == and made a mid-sequence offset throw. Verified on dev before touching anything.
• "Two consecutive <summary> blocks" in DocCommentSource.cs — a misread. The two summaries belong to two different members, StartsTag and IsOpaqueTagOpener, one each. The producer solution builds clean, warnings-as-errors included.

Fixed

• Two abbreviated test names in the §5.1 design doc. …_present_and_empty_entry_window_is_not_absent_and_does_not_fall_back matched no test and could not be grepped, in a document whose stated purpose is pinning behaviour to tests. Both references now carry the full Class.Method name. The review flagged one; the second, in the C5 row of the coverage table, is the same defect and would have aged the same way.
• ConceptSearch.TopDiversified had no null guard while its sibling TopDiversifiedBy throws ArgumentNullException — a null list reached scored.Count and came back out as a NullReferenceException. Guarded, with a test confirmed red before the fix.
• Mismatched <para> tags in BundleWriter.IsWithinPrefixRoot's doc comment: a second <para> opened before the first closed, both closed at the end. Harmless today — producers/ inherits TreatWarningsAsErrors but never sets GenerateDocumentationFile, so nothing parses that XML — which is exactly why it went unnoticed.
• A duplicated assertion in CheckTests: Scanner.Cache asserted absent twice, ten lines apart.

OKF4net: 1273 tests green. Producer: 675 green. Both solutions dotnet format-clean.

🤖 Generated with Claude Code

[Copilot]

🟢 Approval recommended

The changes are small, targeted, consistent with existing contracts, and the updated test references and XML-doc fix are verifiably correct.

Pull request overview

This PR closes remaining Copilot review findings that were left open after earlier PRs merged, focusing on correctness/consistency fixes (argument validation, doc XML correctness), plus cleanup of a redundant test assertion and accuracy of test references in a design doc.

Changes:

• Add an explicit null-argument guard to ConceptSearch.TopDiversified and a targeted unit test to ensure it throws ArgumentNullException instead of surfacing a NullReferenceException.
• Fix malformed XML doc comment <para> nesting in BundleWriter.IsWithinPrefixRoot’s documentation.
• Remove a duplicated assertion in producer tests and correct abbreviated test-name references in a spec/design markdown document.

File summaries

File	Description
tests/OKF4net.Tests/ConceptSearchTests.cs	Adds a unit test asserting TopDiversified rejects a null list with ArgumentNullException.
src/OKF4net/ConceptSearch.cs	Adds ArgumentNullException.ThrowIfNull(scored) to match TopDiversifiedBy’s null-handling contract.
producers/tests/OkfProducer.Tests/Generation/CheckTests.cs	Removes a duplicated “does not contain” assertion.
producers/src/OkfProducer.Core/Generation/BundleWriter.cs	Fixes XML doc comment <para> tag nesting so the summary is well-formed.
docs/superpowers/specs/2026-09-01-okf-per-entry-usage-window-design.md	Replaces abbreviated/un-greppable test references with full Class.Method names for traceability.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 0
• Review effort level: Lite

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