Emit one stable error for invalid outline projections

[Widthdom]

Summary

• collect unknown --outline-fields values and emit one terminal usage error with the invalid names and valid candidates
• stop empty-selection and all/specific-field validation after that terminal projection error
• preserve deliberate empty CSV behavior plus alias canonicalization and duplicate elimination
• add focused parser and JSON error-envelope coverage

Root cause

ParseOutlineProjectionFields appended an unsupported-field error while parsing and then continued into downstream empty-selection validation. Invalid-only input therefore produced two top-level diagnostics. The parser now records invalid fields and returns immediately after emitting the single consolidated projection error.

Validation

• dotnet build CodeIndex.sln -p:UseSharedCompilation=false — passed, 0 warnings / 0 errors
• dotnet build CodeIndex.sln -c Release --no-restore -p:UseSharedCompilation=false — passed, 0 warnings / 0 errors
• focused Issue Emit one stable error for invalid outline projections #4876 tests on net8.0 and net9.0 — passed
• all net8.0 RunOutline tests — 48 passed
• dotnet format CodeIndex.sln --verify-no-changes --no-restore — passed
• dotnet run --project tools/CodeIndex.Changelog -- check — 46 fragments validated
• CLI smoke tests for one/multiple unknowns, deliberate empty CSV, and valid aliases/duplicates — passed
• post-commit CodeIndex refresh with index . --commits HEAD followed by status --check --json — index_matches_workspace: true

Full-suite note:

• net8.0: 10,807 passed, 7 skipped, with one unrelated existing watcher failure in RunCore_SubprojectObservesAncestorIgnoreFileChanges_Issue4592; its isolated retry passed
• net9.0: 10,333 passed, 420 skipped, with the same unrelated existing Use one membership and invalidation policy for scan, status, and watch #4592 watcher failure; its one allowed isolated retry also failed

No changes for #4592 are included because this PR is scoped to #4876.

Documentation and changelog

• updated the English and Japanese sections of DEVELOPER_GUIDE.md and TESTING_GUIDE.md
• added bilingual fragment changelog.d/unreleased/4876.fixed.md
• did not edit CHANGELOG.md

Adversarial review

Codex reviewed origin/main..HEAD: No blocking/actionable issues found.

Fixes #4876