Harden public API for 0.6.0 release

[joshuajbouw]

Summary

Break all deprecated APIs and harden the public contract before the 0.6.0 release. Since every capsule needs a rebuild (Component Model ABI change from #29), ship all breaking changes in one version.

Breaking changes

Removed

• ipc::publish_bytes, ipc::publish_msgpack, ipc::poll_bytes, ipc::recv_bytes — use ipc::publish, ipc::publish_json, ipc::poll, ipc::recv
• uplink::send_bytes — use uplink::send
• net::bind_unix(path) — use net::bind_unix() (no path arg, kernel pre-binds)
• hooks::trigger(&[u8]) — now trigger(&str) -> String
• process::ProcessRequest struct (dead code)
• SubscriptionHandle::as_bytes() — use .id()
• approval::RiskLevel, approval::ApprovalDecision, approval::ApprovalResult — approval simplified to request(action, resource) -> bool
• rmp-serde dependency (only user was publish_msgpack)
• bytes_to_str helper (all APIs now take &str)

New typed APIs

• ipc::poll() / ipc::recv() return typed PollResult { messages: Vec<Message>, dropped, lagged }
• http::Response exposes status(), headers(), is_success()
• approval::request(action, resource) -> Result<bool> (2 params, returns bool)
• log::trace() added

Simplified

• All log functions return () (was Result — never fails)
• http::Response fields all via methods (consistent encapsulation)
• All handle types standardized: private inner u64, public id() method
• interceptors::poll handler receives typed &ipc::PollResult

Companion PRs

• Core WIT approval cleanup: refactor(approval): remove risk_level and decision from WIT contract astrid#642
• Core WIT risk_level removal: refactor(approval): remove risk_level and decision from capsule-facing WIT contract astrid#641