Avoid content-bearing sorts in line-local find

[Widthdom]

Line-local literal/regex find previously sorted chunk content in SQLite before yielding the first line. It now selects chunk metadata in index order, or in 64-row keyset pages on older indexes, and fetches source content separately per chunk. Overlap ownership, missing-source failures, counts, scan limits and continuation positions are preserved without a schema migration or reindex.

The legacy sort retains only bounded scalar metadata. Legacy pages and the NULL-content compatibility check may still scan file metadata; individual chunk content is still read in full. No peak-memory or latency benchmark is claimed.

Validation:

• Release net8.0: 638 related find, cursor, bounded-output, envelope, response-budget and MCP regression tests passed with the current main integration.
• Release net9.0: 628 passed; 10 existing framework/platform skips.
• make lint passed.
• Shared command-guard tests: 49 passed. Focused Release chunk-read regressions: 4 passed on each of net8.0 and net9.0 after the latest main integration.
• Many-chunk regressions cover current/read-only legacy query plans, bounded pages, reversed insertion, overlaps, missing/live source, NULL content, literal/exact/trigram/regex variants and row/count replay.
• Real-index query plan: the old query uses USE TEMP B-TREE FOR ORDER BY; the new ordered query uses idx_chunks_file_start_chunk_nonnull without a temporary sort.
• dotnet run --project tools/CodeIndex.Changelog -- check and git diff --check passed.
• Codex adversarial reviews: No blocking/actionable issues found. Coverage includes the bounded legacy metadata sort, formatting fix, and current main integration; the merge review verified that both parents' bilingual documentation was preserved and non-conflicting files match their respective parents.
• Rebuilt the local CLI, refreshed the root index after the commit, and verified both root and workspace status checks are healthy.

Documentation is updated in English and Japanese in docs/find-scan-controls.md and TESTING_GUIDE.md. Changelog fragment: changelog.d/unreleased/5415.fixed.md.

Fixes #5415