Fix excerpt range semantics and EOF recovery

[Widthdom]

Summary

• Preserve the caller's requested excerpt range separately from the context-expanded effective range, and expose the indexed total_lines contract in CLI/MCP results.
• Add explicit --end eof and --clamp recovery modes while retaining strict behavior for numeric ranges that exceed EOF.
• Treat zero and negative coordinates as E020_LINE_OUT_OF_RANGE, keep the numeric-only --end-line alias strict, and preserve rightmost-option semantics for repeated end aliases.
• Document the CLI/MCP behavior in both English and Japanese and add a bilingual changelog fragment.

Root cause

Excerpt range normalization reused context-expanded values as the requested coordinates, which erased the original request. Numeric overshoot also flowed into unsafe range construction, while coordinate validation classified zero inconsistently and offered no explicit EOF/clamping contract.

Validation

• dotnet format CodeIndex.sln --no-restore --verify-no-changes
• dotnet run --project tools/CodeIndex.Changelog -- check (49 fragments validated)
• dotnet build CodeIndex.sln -c Release --no-restore -p:UseSharedCompilation=false (0 warnings, 0 errors)
• Integrated excerpt/CLI-schema/inspect/MCP/golden regressions: .NET 8: 100 passed; .NET 9: 99 passed, 1 platform-dependent test skipped
• Focused Issue Separate requested and effective excerpt ranges and add explicit EOF/clamp modes #4877 regressions after the second review fixes: 6 passed on .NET 8 and 6 passed on .NET 9
• Local CodeIndex status: 1,275/1,275 files matched; index and reference graph fresh and complete
• Full-suite exploratory runs before final integration found one unrelated watcher timing failure that passed in isolation and one stale inspect expectation that was corrected; all affected and integrated targets passed afterward on both TFMs.

Adversarial review

Two Codex review rounds completed. The first round found invalid completion literals and missing empty-result MCP range metadata; the second found repeated end-alias precedence and invalid start-overshoot recovery advice. All actionable findings were fixed and covered by regressions.

Documentation / changelog

• Updated USER_GUIDE.md and TESTING_GUIDE.md in both English and Japanese.
• Added changelog.d/unreleased/4877.fixed.md.

Follow-up candidates

• None.

Fixes #4877