Add bounded multiline regex find to CLI and MCP

[Widthdom]

Summary

Add opt-in bounded multiline regex matching to indexed find and MCP find / find_in_file. The adjacent-using example now returns one span from 1:1 to exclusive 2:24 with --regex --multiline; ordinary regex remains line-local.

• Share indexed window scanning, UTF-16 endpoints, non-overlapping count/row continuation, and explicit resource limits across CLI and MCP.
• Preserve span and completeness metadata through projections and response byte limits; reject unsupported semantic, focus, context, and row-only format combinations.
• Document window semantics and limits in English/Japanese, update help and guides, and add changelog.d/unreleased/5399.added.md.

Validation

• Debug and Release builds passed. Full Release suites passed: net8.0 12,764 passed / 9 skipped; net9.0 12,270 passed / 435 skipped; zero failures on both targets. The final follow-up changes formatting only and passes make lint.
• Final multiline regression tests: 6 passed on each target, including all review regressions.
• After merging current main: 14 combined multiline/Python-origin regressions passed on each target.
• Focused find, pagination, MCP, output, and argument tests: net8.0 1,462 passed; net9.0 1,333 passed, 84 skipped.
• Adjacent-using reproduction, changelog fragment validation, make lint, and git diff --check passed.
• Codex adversarial review of origin/main..HEAD: two rounds completed; all four findings addressed with regression coverage.
• Root and workspace index checks passed after refreshing the committed changes.

Follow-up

• Return structured JSON for bounded find cursor validation errors #5412
• Avoid content-bearing temporary sorts in line-local find chunk reads #5415

Fixes #5399