Stabilize symbol ordering and reference dedupe

[Widthdom]

Summary

• Add deterministic start_column and symbol id tie-breaks for definition symbol searches.
• Add deterministic hotspot tie-breaks by path, line, name, kind, and symbol id, and document the MCP ordering.
• Include container kind/name in reference dedupe keys so same-position references from different enclosing symbols are preserved.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter "BuildReferenceDedupeKey|AddTypeReferenceSegment_DedupesWithinFileAndLanguageOnly"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter "SearchSymbols_BreaksSameLineTiesByStartColumn|GetSymbolHotspots_BreaksEqualCountsByPathLineNameKind|GetSymbolHotspots_RanksRealCallsAboveManyLowerWeightSubscribeEdges"
• dotnet run --project tools/CodeIndex.Changelog -- check
• Full dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false was attempted; it failed because test-local git commit commands inherited the local SSH signing key and could not unlock /Users/widthdom/.ssh/id_ed25519_signing.
• Re-running full tests with GIT_CONFIG_COUNT=1 GIT_CONFIG_KEY_0=commit.gpgsign GIT_CONFIG_VALUE_0=false was attempted; MSBuild failed in this sandbox with System.Net.Sockets.SocketException (13): Permission denied while starting out-of-proc nodes.

Documentation and changelog

• Added changelog.d/unreleased/2015.fixed.md
• Added changelog.d/unreleased/1934.fixed.md
• Added changelog.d/unreleased/1974.fixed.md

Follow-up candidates

• Existing index: full scan stalls extracting SqlReferenceExtractor.cs symbols #2710 tracks the E013_INDEX_EXTRACTION_STALLED full-scan failure observed while refreshing the local index.

Fixes #2015
Fixes #1934
Fixes #1974