cs2gs + LS: preserve C# tuple element names + completion (ADR-0172 Phases C+D, #3501)

[DavidObando]

Summary

Stacked on #3622 (named tuple elements, Phases A+B) — Phase C of ADR-0172: cs2gs stops erasing C# tuple element names, delivering the #3501 readability win across the ~830 named-tuple declaration lines in the self-migration corpus.

• Types print name-first: (int Line, int Column) → (Line int32, Column int32) (CSharpTypeMapper keeps Roslyn's TupleElements names; a default ItemN-at-position counts as unnamed; the ADR-0115 §B.4 name-drop Info diagnostic is retired). Names survive the nullability-promotion rebuild sites too (oblivious promotion, whole-type nullable wraps).
• Literal labels survive: (Count: 3, Name: "three") via NameColon.
• Named element access stays by-name: item.Price * item.Quantity round-trips verbatim where it previously lowered to item.Item2 * item.Item3; default positional .ItemN access still normalizes positionally.
• TupleTypeReference / TupleLiteralExpression gain optional element-name lists; the printer renders both forms.

Verification

• New Adr0172NamedTupleTranslationTests (6): name-first types, by-name access, positional access unchanged, literal labels, unnamed tuples unchanged, names inside generics — every snippet re-binds through the real gsc (exercising the G#: named tuple elements — Phases A+B: front end + metadata interop (ADR-0172, #3501) #3622 front end).
• Existing tuple-behavior tests updated to the named surface (Issue1914 alias, Issue2469 nullability ×6, Issue3615 receiver probe, oblivious-promotion sinks); coverage-matrix notes moved into the inventory and docs/cs2gs-coverage-matrix.md regenerated via cs2gs coverage --write.
• Full Cs2Gs.Tests green (2454 tests; the two last local failures were a stale out/bin/Release gsc found by FindCompiler — rebuilt — and a stale expectation).
• Corpus diagnostic-run 18/20 green with the branch gsc (CompileGap-Library is the known wontfix; L3-Library's test-parity failure is the documented stale-SDK-nupkg trap — the parity stage rebuilds with the packaged gsc, which predates ADR-0172; CI builds the SDK from the same commit). Translated L1 now reads total += item.Price * item.Quantity with List[(Name string, Price int32, Quantity int32)].

🤖 Generated with Claude Code

https://claude.ai/code/session_01Nng28yiBdPVdeML7mSZphs

[DavidObando]

Also carries Phase D (LS): tuple-typed receivers' dot-completion now offers declared element names first, then the positional ItemN spellings, with element types as detail. Full LanguageServer.Tests green (486). Both branches rebased onto main after #3621's squash-merge.