feat(sdk): expose heterogeneous readiness polling

[joshuajbouw]

Linked Issue

Closes #64

Summary

Expose the canonical heterogeneous readiness surface through safe Rust SDK wrappers so capsules can multiplex IPC subscriptions, listeners, and streams without timeout polling.

Changes

• add an opaque io::Pollable wrapper with ready and block
• add io::poll with SDK-side empty-list and 256-handle validation
• expose readiness subscriptions on IPC subscriptions, Unix listeners, and network streams
• document borrowed-resource drop ordering and export the module through the prelude
• add SDK validation coverage and an [Unreleased] changelog entry

Verification

• cargo check --workspace --offline
• cargo test -p astrid-sdk --offline (39 unit tests plus smoke and doc tests)
• GitHub CI Clippy passed for the workspace
• cargo fmt --all -- --check
• git diff --check

Checklist

• Linked to an issue
• CHANGELOG.md updated