fix(shared-memory): copy envelopes at transport handoff

[Steve-XYZ]

Summary

Fixes #3015 by giving Shared Memory receivers a new Envelope instance at the transport handoff boundary instead of retaining the sender's pooled envelope.
This follows the intended ownership model: the sender may recycle its pooled envelope after SendAsync() completes, while each asynchronous receiver owns an independent envelope wrapper.

Changes

• add a Shared Memory-local envelope copy helper;
• create a distinct envelope copy per topic subscription;
• copy direct subscription sends before posting to the receiver;
• preserve message, transport metadata, headers, context fields, and custom metric tags;
• intentionally exclude sender, pool, persistence, batching, and receiver runtime lifecycle state;
• add a deterministic regression test proving receiver copies remain valid after the original sender envelope is recycled.

Validation

• deterministic regression test: 10/10 independent runs passed;
• focused CoreTests Shared Memory coverage: 14 passed;
• scheduled/context Shared Memory regression: passed;
• CoreTests net10.0: 1861 passed, 2 skipped;
• focused request/reply and correlation tests passed when run independently.
  The combined Shared Memory compliance selection has pre-existing cross-test interference in can_send_from_one_node_to_another_by_publishing_rule; both inline and buffered cases pass in isolation.

[jeremydmiller]

@Steve-XYZ Thanks for taking that on! Frankly, that was probably never something I was going to get around to