fix: isolate console and agent lifecycle ownership

[dust-life]

What changed

This fixes REM lifecycle ownership so independent Consoles and Agent generations do not close or remove one another.

• Scope Console.Close to the root tree owned by that Console.
• Keep shared parent transport ownership at the root; closing a child no longer closes the shared transport.
• Serialize Fork publication with parent and child lifecycle locks.
• Reject closed Agents and different pointers that collide on the same ID.
• Remove registry and parent-child entries by exact pointer, protecting replacement generations.
• Prevent closed children or late cleanup from republishing or deleting a newer generation.
• Preserve a stopped child alias for a later generation.
• Add regression coverage for duplicate Fork, Fork versus Close, Add versus Close, closed Add, generation replacement, alias rebuild, and independent Console transports.

Validation

• go test -race ./agent
• go test -race ./runner -run 'Test(Console|Agent|.*Ownership|.*Fork|.*Close|.*Restart|.*Reconnect)' -count=1 -timeout=120s
• go vet ./agent ./runner
• Real loopback service restart/reconnect test passes for the candidate and clean base.

The full Runner race matrix is not claimed as green. The clean base reproduces the simplex address race, memory helper race/failure, and Unix listener readiness failures. The full matrix also has an unresolved timeout and another Bridge race that were not individually compared with the base; those are outside this focused lifecycle change.

The change is limited to lifecycle ownership and regression tests. It does not add protocol or network features.