Postgres: JSON and custom payload serializers

[Skymly]

Summary

• Add Postgres payload serializers (STJ default for non-string T, Mqtt/Nats-shaped PostgresPayloadSerializers registration)
• Emit typed FromListen<T> / FromNotify<T> while keeping the string golden-path overloads unchanged
• Cover R3 + Reactive E2E (typed JSON + custom colon-delimited serializer) and generator Verify snapshots

Closes #158

Test plan

• dotnet run --project build/_build.csproj -- --target Test --configuration Release --test-domains postgres
• Confirm existing string Listen/Notify E2E still pass on R3 and Reactive
• Confirm CI postgres matrix / pack jobs green

---

Note

Medium Risk
Expands the public contract and NOTIFY wire format for typed interfaces (JSON by default); deserialization errors surface on the observable stream. Trimming/AOT callers must heed RequiresUnreferencedCode on JSON paths—mitigated by preserving string behavior and test coverage.

Overview
Adds payload serialization for PostgreSQL LISTEN/NOTIFY: PostgresPayloadSerializers (global default + per-type registration), with UTF-8 strings and System.Text.Json for other types by default, and hooks for custom wire formats.

R3 (PostgresObservable) and System.Reactive (SystemReactivePostgresAdapter) gain generic FromListen<T> / FromNotify<T> that serialize/deserialize through that pipeline; existing string overloads are unchanged.

The source generator now allows non-string payload types on [Listen] / [Notify] and emits the matching generic bridge calls (verified snapshots for R3 and Reactive). E2E and unit tests cover JSON round-trips and a custom colon-delimited serializer.

^{Reviewed by Cursor Bugbot for commit a3eeda1. Configure here.}