Add resumable cursors to NDJSON terminals

[Widthdom]

Summary

• Add reusable response:v2 cursors to truncated raw NDJSON terminal records for search, symbols, and files.
• Bind continuation to the command, query, filters, ordering, offset, and index generation, and advance by the rows actually emitted under the byte budget.
• Fail closed with machine-readable reasons when selectors, zero-progress byte caps, generation changes, or the 10,000-row replay window make continuation unsafe.
• Keep byte-trimmed envelope and nested metadata.stream_terminal cursors aligned so replay has no gaps or duplicates.

Validation

• dotnet build CodeIndex.sln -c Release --no-restore -p:UseSharedCompilation=false --disable-build-servers -m:1 /nodeReuse:false --nologo --verbosity:minimal (0 warnings, 0 errors)
• Issue-focused Release tests on net8.0: 6 passed, 0 failed
• Issue-focused Release tests on net9.0: 6 passed, 0 failed
• Full Release net8.0 suite: 12,157 passed, 9 skipped, 0 failed
• Full Release net9.0 suite: 11,670 passed, 433 skipped, 0 failed
• dotnet format CodeIndex.sln --verify-no-changes --no-restore --verbosity minimal
• dotnet run --project tools/CodeIndex.Changelog -- check
• Root and workspace index health checks against the locally built cdidx

Documentation and changelog

• Updated the English and Japanese NDJSON cursor contract in USER_GUIDE.md, DEVELOPER_GUIDE.md, and TESTING_GUIDE.md.
• Added bilingual changelog fragment changelog.d/unreleased/5230.fixed.md.

Adversarial review

• Round 1: addressed unsafe cursor advertisement at the pagination boundary and concurrent index-generation drift.
• Round 2: addressed byte-trimmed nested cursor gaps and cursors whose unchanged page size could not be replayed within the safety window.
• Added regression coverage for every actionable finding; no findings remain unresolved.

Follow-up candidates

• None.

Fixes #5230