runtime: keep managed keyed posts outside callback ingress

[cpunion]

Summary:

• post scheduler-owned keyed semaphore/notify completions directly through the managed source/request/doorbell path
• reserve TargetIngress leases for real external C/host producers that require strong callback join
• add a source architecture gate preventing the managed path from reacquiring external ingress

Root cause:
The sync acceptance workload could return main while a managed keyed post was suspended in the external callback tail. Route 1 then reached backend retirement with TargetIngress state sealed+count=1 and waited forever. The llvm-coro baseline reproduced this in 30-40 runs at GOMAXPROCS=8; both baseline and candidate stacks stopped in coroNativeFleetRetireBackendV1 -> Pipe.WaitBounded.

Validation:

• fresh LLGo sync stdlib acceptance compile/link/run
• 250 consecutive runs of the independently built artifact at GOMAXPROCS=8
• runtime architecture source gate
• stdlib acceptance configuration and fixture tests

[gemini-code-assist]

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