feat(sdk): surface publisher principal on ipc::Message

[joshuajbouw]

Summary

Adds the receive-side half of the ipc-publish-as feature. Canonical WIT PR unicity-astrid/wit#4 added principal: option<string> to the ipc-message record; this PR consumes it.

Motivation

unicity-astrid/astrid#735 installs an invocation context from the first message of an ipc::recv batch — host calls the subscriber makes after recv attribute to that publisher. Covers single-message subscribers. Breaks down for multi-message batches where messages 1..N may have different publishers: the subscriber sees msg[0]'s principal as the invocation context for all of them.

Surfacing principal per-message lets subscribers route / log / authorize correctly without relying on that invocation-context shortcut.

Changes

• Bumps contracts/ submodule to unicity-astrid/wit#4's merge commit.
• Re-runs scripts/sync-host-wit.sh — astrid-sys/wit/astrid-capsule.wit picks up the new field. The wit-bindgen-generated astrid_sys::astrid::capsule::ipc::IpcMessage now carries principal: Option<String>.
• Surfaces the field on the SDK's astrid_sdk::ipc::Message struct, with docstring describing the Some/None semantics.
• envelope_to_poll_result plumbs the new field from the WIT envelope through to the user-facing PollResult.

Until the kernel publish path stamps the field (separate follow-up PR on unicity-astrid/astrid), the field is None at runtime for every message — contract is now in place and the SDK surface is ready to consume real data the moment the kernel side lands.

Backward compatibility

option<string> by construction. Older guests reading envelopes from a newer kernel ignore the field; newer guests see it. Record evolution under the wasmtime component model handles the rest.

Test plan

• cargo build --workspace — clean.
• scripts/sync-host-wit.sh --check — passes.
• Rebased onto 0.6.1's tip; no test regressions.

[gemini-code-assist]

Code Review

This pull request introduces a principal field to the Message struct within the IPC module, allowing subscribers to identify the publisher of individual messages in multi-message batches. This change includes updates to the SDK's internal mapping logic and documentation. A review comment suggests using a fully qualified repository reference for a pull request mentioned in the changelog to ensure correct cross-repository linking.

[gemini-code-assist]

The reference to kernel PR #735 is ambiguous because it refers to a pull request in a different repository (unicity-astrid/astrid). To ensure clarity for users and to allow GitHub to correctly link the reference, consider using the fully qualified reference unicity-astrid/astrid#735.

[joshuajbouw]

Superseded by #44 (per-domain WIT migration).

The per-domain astrid:ipc/host shape ships a typed PrincipalAttribution (Verified(String) / Claimed(String) / System) instead of Option<String> — strictly richer than what this PR proposed:

pub struct Message {
    pub topic: String,
    pub payload: String,
    pub source_id: String,
    pub principal: PrincipalAttribution,  // ← was the goal here
}

The Verified vs Claimed distinction (kernel-verified principal vs uplink-asserted) is what the original "log / route / authorize correctly" motivation actually needed, and Option<String> couldn't express it.