Make partial find scans resumable

[Widthdom]

Summary

• return stable find --all continuation cursors at match or line boundaries for result limits, scan caps, and paged counts
• bind cursors to the query, row/count scan mode, source position, and index generation, with typed malformed, mismatch, and stale errors
• preserve deterministic no-duplicate/no-gap pagination for same-line matches, Unicode, large lines, byte budgets, cancellation, and regex timeouts
• reject count cursors combined with row-only bounded-response controls and keep resumed count-page authority explicit

Validation

• dotnet build CodeIndex.sln -c Release --no-restore -p:UseSharedCompilation=false (0 warnings, 0 errors)
• dotnet build CodeIndex.sln -c Debug --no-restore -p:UseSharedCompilation=false (0 warnings, 0 errors)
• expanded find/bounded-response tests: net8 66 passed; net9 65 passed, 1 intentional practical-budget skip
• full Release suite during implementation: net9 10,240 passed / 420 skipped; net8 10,712 passed / 7 skipped with 2 unrelated flaky failures, both passing when rerun in isolation
• dotnet run --project tools/CodeIndex.Changelog -- check (33 fragments validated)
• dotnet format CodeIndex.sln --no-restore --include ...
• git diff --check
• local status --check --json reports index_matches_workspace: true

Adversarial review

Two Codex adversarial-review rounds were completed (the workflow maximum). Round 1 found and fixed cancellation propagation, out-of-range resume-line validation, and high-ordinal UTF-8 resume performance. Round 2 found and fixed row/count cursor mode separation, count/bounded-control validation, and resumed count-page authority semantics. No known actionable findings remain.

Documentation and changelog

• updated docs/find-scan-controls.md in English and Japanese
• added changelog.d/unreleased/4863.fixed.md

Follow-ups

None.

Fixes #4863