fix: close packet channel on non-temporary errors

[leno23]

Summary

• Close PacketSource.Packets() when NextPacket returns an unknown non-temporary error instead of retrying forever.
• Match known retry/terminal errors through a small Unwrap-aware helper so wrapped errors are classified consistently without requiring errors.Is (the module still declares Go 1.12).
• Add a regression test covering the starvation case where Packets() previously never closed after a persistent non-temporary read error.
• Update Packets documentation to describe the non-temporary error behavior.

Related Issues / Closes

Closes #1143

Testing

• go test . -run TestPacketSourcePacketsClosesOnNonTemporaryError -count=1 (passes)
• go test . (passes)
• go vet . (passes)
• git diff --check (passes)

Additional validation:

• I also ran go test ./...; it fails in unrelated platform/environment-dependent packages on this macOS runner:
  • examples/afpacket: Linux-only afpacket build constraints exclude all Go files.
  • pfring / examples/pfdump: missing system header pfring.h.
  • pcap: TestPCAPGoNgWrite reports could not read first packet: Read Error.

AI assistance disclosure

This draft PR was prepared with AI assistance from Hermes Agent. I reviewed the issue context, implemented the minimal change, and ran the validation listed above.

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