feat(stream): implement declared event ingress - #298

Summary

Implements the stream subsystem specified by #287 as its dependent implementation PR.

• admits typed nested stream declarations with command, argv, or external ingress
• lowers launched streams directly to lifecycle-coupled derived exec companions
• adds source-preserving st2 stream add/rm authoring
• adds bounded, idempotent st2 event emit ingress with keyed and keyless supersession
• publishes ordinary inbox records and reuses existing archive/DING safety
• resolves the st2 VRS implementation delta

Verification

PASS:

• local nix flake check --print-build-logs (all 9 x86_64-linux checks)
• formatting and workspace all-target checks
• event E2E: 14/14
• Agent Spec discovery: 55/55
• message CLI: 18/18
• authoring, lifecycle, wire, and invariant proofs

Canonical failure-capable E2E proof is in compoundingtech/evals#79, exact-pinned to 2fa21f12212777ee46857d6eb3eb13db1b4ba64b; its stream cell passes 8/8 and complete corpus preflight passes.

Stack

• docs(vrs): add the 04-stream subsystem intent layer #287 - VRS/design
• feat(stream): implement declared event ingress #298 - implementation and executable proofs

The branch base preserves the dependency. GitHub stack metadata was removed after it repeatedly reported a false conflict for a direct-descendant head.

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