Fix the 5.2.0-beta.1 findings: Npgsql floor, ConnectionBoundSchemes builder, CS0436

[windischb]

Why

amZettel tested 5.2.0-beta.1 on all clients and reported three findings. One of them blocks it: the Postgres backplane.

What

1. Postgres backplane demanded Npgsql 10 on net10.0 (blocking)

• The central Npgsql version follows the target framework (8.0.6 / 9.0.4 / 10.0.3), and the backplane package inherited it as its minimum. A net10.0 app held on Npgsql 9 by Marten therefore got NU1605.
• The backplane now builds against Npgsql 8.0.6 on every framework (VersionOverride). The nuspec asks for Npgsql >= 8.0.6 in all three groups. The code only needs NpgsqlDataSource and LISTEN/NOTIFY.

2. ConnectionBoundSchemes unreachable from the options builder

• AddSignalARRR hands out the builder, which had no way to reach the list, so the documented options.ConnectionBoundSchemes.Add(...) did not compile.
• New: WithConnectionBoundSchemes(params string[]). It de-duplicates names and rejects empty ones. The authorization guide uses it.

3. CS0436 in test projects with the API's InternalsVisibleTo

• The generator emitted referenced-assembly ("Ref") proxies even when the referenced assembly had generated the same proxy itself and that proxy is visible through InternalsVisibleTo.
• It now skips a contract whose proxy exists in the referenced assembly and is accessible from the current compilation. The referenced assembly already registers it in its own module initializer.

Verification

• Unit tests: 290/290 on net8.0, net9.0 and net10.0, including new builder tests.
• Integration tests on net10.0: 108/108. The backplane integration tests (Redis + Postgres, via Docker) pass 46/46 on both net10.0 and net8.0.
• Regression for item 3: Tests.SharedModels now grants InternalsVisibleTo to the integration tests. Without the generator fix that build fails with three CS0436 errors; with the fix it is clean.
• Consumer check for item 1: a packed backplane in a net10.0 app pinned to Npgsql 9.0.4 restores and builds cleanly, and Npgsql resolves to 9.0.4.

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