v0.4.0 — first binaries, and the cold run's fixes

• Admitted untracked traffic is accepted (#142, #154). A rule's accept also wrote the connection mark, and the kernel ends a rule at that write for a packet it does not track, so admitted untracked traffic fell through to the terminal rule. IPv6 neighbor discovery is never tracked: a rule admitting link-local ICMPv6, the simulation review's own remedy, recorded neighbor discovery as would-block in simulation and dropped it under enforcement, which breaks IPv6 neighbors. Each rule's accept is now paired with one that admits the same traffic unmarked when it is untracked; tracked traffic is marked and attributed exactly as before, and the two modes still differ in the terminal rule alone (ADR-0020 as amended). Agent-side only: no migration and no wire change. Upgrade agents before enforcing on a host with IPv6.
• The connection-mark mask, as nft list prints it (#143, #154). A listed rule shows ct mark & 0x000Nffff | 0x000N0000, which read as a mask leaking region bits. It is nft's reading of the kernel's mask-and-xor form of the rendered (ct mark & 0x0000ffff) | 0x000N0000, the same write; the rendering is unchanged, and tests now pin the rendered mask and the installed bytecode. The promotion guide and ADR-0020 say how the listing reads.
• A quiet workload is no longer called silent (#144). The agent ships only the flow windows that hold something, so a workload that receives little inbound traffic — a bastion, a standby replica — goes minutes between reports while its agent runs and observes. v0.3.0's silence derivation read every such stretch longer than two windows as an agent that was not reporting, and wrote agent_silent gaps for evidence that was never lost; the simulation review drew them as one interval spanning the hour and failed its verdict. Every heartbeat already carries the agent's uptime, so the control plane now records the process start each heartbeat implies (migration 00016) and derives no gap for a silence one agent process demonstrably ran through: one that started before the previous report and was still heartbeating within a window of the arriving one (ADR-0019 as amended). A stopped agent still derives its gap with the same bounds — a restarted process's heartbeats place its start after the silence began, and a dead process's last heartbeat is too old to cover the report that ends it — and a quick restart still records nothing. Gaps v0.3.0 already wrote for quiet stretches stay in place, since nothing stored tells them from an outage, and age out with flow retention. The agent is untouched; the migration makes the next release v0.4.0.