Fix CLI and MCP input bounds

[Widthdom]

Summary

• Added pre-split bounds for repo map sections, status --check scopes, visibility filters, query path filters, index symbol-kind filters, and MCP tool allow/deny CSV input.
• Clamped index --parallelism / CDIDX_INDEX_PARALLELISM to 16 workers with warnings for oversized values.
• Added focused regression tests and bilingual changelog fragments for each requested issue.

Validation

• dotnet build src/CodeIndex/CodeIndex.csproj
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~ParseArgs_MapSectionsRejects|FullyQualifiedName~ParseArgs_StatusCheckScopesRejects|FullyQualifiedName~ParseArgs_VisibilityFiltersReject|FullyQualifiedName~ParseArgs_SymbolKind|FullyQualifiedName~McpToolFilter_Parse|FullyQualifiedName~ParseArgs_PathFiltersReject|FullyQualifiedName~Parallelism"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Codex adversarial review completed. Findings were addressed for the MCP tool allow/deny env filters should cap CSV length and entry count #2905 invalid allowlist fail-open path and the MCP tool allow/deny env filters should cap CSV length and entry count #2905 changelog wording; final targeted review reported no blocking/actionable issues.

Notes

• No requested issue was skipped; none had work-start comments before this branch's work-start comments were posted.
• Removed ignored/untracked .review-tmp/ review fixtures from the worktree.
• Full suite validation separately exposed DbPathResolverTests.ResolveDataDirForQuery_* failures unrelated to this PR; filed DbPathResolverTests ResolveDataDirForQuery tests resolve temp root data dir #2953 and left this PR scoped.
• AGENTS.md, CLAUDE.md, and AGENT_GUIDE.md are not modified by this PR.

Related: #2953

Fixes #2914
Fixes #2913
Fixes #2912
Fixes #2906
Fixes #2905
Fixes #2911
Fixes #2904