chore: NativeAOT / Trim compatibility audit and annotations for all Pulse packages

[samtrion]

User Story

As a developer publishing a NativeAOT or trimmed application using Pulse, I want Pulse to be fully trim-compatible and NativeAOT-safe, so that I can publish single-file or NativeAOT binaries without trim warnings or runtime failures.

---

Background

This is the lowest priority item, to be implemented last after all other features are complete. No new features are introduced — this is pure annotation, configuration, and source generator work.

---

Requirements

• Enable ILLink trimmer warnings for all Pulse projects; document all [RequiresUnreferencedCode] sites.
• Add [DynamicallyAccessedMembers] annotations to all Type parameters used for reflection (handler resolution, interceptor pipeline, event dispatcher).
• Extend the source generator to emit ILLink.Descriptors.xml covering all generated handler registration classes.
• Add [JsonSerializable] source-generation (System.Text.Json) for all internal Pulse DTOs: Outbox payload, AuditRecord, CommandDeadLetterEntry.
• Add GitHub Actions CI step: dotnet publish -r linux-x64 -p:PublishAot=true for a test project; fail on any trim warning.

---

Acceptance Criteria

• Zero [RequiresUnreferencedCode] warnings in Pulse package builds.
• Source generator emits ILLink.Descriptors.xml covering all handler types.
• JsonSerializerContext covers all internal serialized DTOs.
• CI NativeAOT publish step passes without warnings.

---

Out of Scope

• User-defined handler types (the user's application code).