Report search selector population accounting

[Widthdom]

Summary

• separate source, selected, returned, selector-omitted, and limit-omitted search populations
• preserve selector accounting for zero-result issue drafts and total-limit-exhausted recipe queries
• keep compact and array-envelope returned counts accurate after hard byte-cap trimming
• reject output modes that cannot represent row selectors instead of silently ignoring them

Root cause and impact

Selector stages and output limits were previously collapsed into the same result totals, and some output shapes either discarded selector accounting or retained pre-trim counts. Machine consumers can now audit selector behavior consistently across supported search and recipe outputs.

Validation

• dotnet format CodeIndex.sln --no-restore --verify-no-changes
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-restore
• related Release tests on .NET 8 and .NET 9: 16 passed per target
• full Release solution tests: .NET 8 — 10,595 passed, 7 skipped; .NET 9 — 10,122 passed, 419 skipped
• dotnet run --project tools/CodeIndex.Changelog -- check
• two Codex adversarial-review rounds; all blocking/actionable findings addressed

Documentation and changelog

• updated English and Japanese sections in USER_GUIDE.md and DEVELOPER_GUIDE.md
• added changelog.d/unreleased/4843.fixed.md

Follow-ups

None.

Fixes #4843