Adopt Microsoft.CodeAnalysis.PublicApiAnalyzers to track the public API surface

[thomhurst]

Problem

The library's public surface has drifted without oversight — incidental members shipped (wrapper-struct equality operators, Deconstruct), and behavioural contracts (input enumerated once vs lazily) changed without any tripwire. PR #332 removes surface as part of v4; nothing prevents the next accidental addition or removal.

Proposal

Add Microsoft.CodeAnalysis.PublicApiAnalyzers to the library project:

• PublicAPI.Shipped.txt seeded from the v4 surface at release.
• PublicAPI.Unshipped.txt for in-flight changes.
• CI fails on undeclared surface changes; reviewing a PR that touches the API becomes a one-file diff.

Acceptance criteria

• Analyzer active on all TFMs, zero diagnostics at baseline.
• CONTRIBUTING note: how to update the txt files when intentionally changing the API.