feat(capsule): short-circuit interceptor chain with veto semantics

[joshuajbouw]

Linked Issue

Closes #585

Summary

Evolve the interceptor priority system from "ordered execution of all matching interceptors" to a middleware chain with short-circuit/veto capability. Interceptors return Continue, Final, or Deny to control the chain. A guard at priority 10 can veto an event before the core handler at priority 100 ever sees it.

Changes

• Add InterceptResult enum (Continue/Final/Deny) to capsule.rs with wire format decoder
• Change invoke_interceptor return type from CapsuleResult<Vec<u8>> to CapsuleResult<InterceptResult> across all trait implementations (Capsule, ExecutionEngine, WasmEngine, McpEngine)
• WASM engine: decode guest bytes via InterceptResult::from_guest_bytes() — backward compatible (empty bytes = Continue)
• Dispatcher: multi-interceptor events run sequentially in priority order as a chain; single-interceptor events use existing per-capsule queue fast path
• Update hooks trigger (sys.rs) to handle all three result variants
• Tests for Deny short-circuit, Final short-circuit, and wire format round-trip

Prior Art

• Bevy ECS — system ordering with run conditions
• Express.js / Koa — next() middleware chain
• Envoy/Istio — HTTP filter chain with early termination
• Linux netfilter — NF_ACCEPT / NF_DROP / NF_QUEUE
• DOM events — stopPropagation() / preventDefault()

Test Plan

Automated

• cargo test --workspace passes (24 dispatcher tests including 3 new chain tests)
• No new clippy warnings

Manual

• Deploy guard capsule at priority 10 with Deny, verify core at 100 never fires
• Deploy cache capsule at priority 30 with Final, verify core at 100 never fires
• Existing capsules (no discriminant byte) continue working unchanged

Checklist

• Linked to an issue
• CHANGELOG.md updated under [Unreleased]

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a significant enhancement to the interceptor system by implementing a middleware chain with short-circuit capabilities. This change allows for more flexible and powerful control over event processing, enabling features like input guards and caching layers. The introduction of the InterceptResult enum and the corresponding logic in the dispatcher and WASM engine provides a robust and backward-compatible mechanism for managing the interceptor chain.

Highlights

• Interceptor Chain Evolution: The interceptor priority system has been evolved into a middleware chain with short-circuit/veto capability, allowing interceptors to return Continue, Final, or Deny to control the chain.
• InterceptResult Enum: An InterceptResult enum (Continue/Final/Deny) has been added to capsule.rs with a wire format decoder to manage the flow of the interceptor chain.
• WASM Engine Compatibility: The WASM engine now decodes guest bytes via InterceptResult::from_guest_bytes(), ensuring backward compatibility where empty bytes are treated as Continue.
• Dispatcher Updates: The dispatcher has been updated to run multi-interceptor events sequentially in priority order as a chain, while single-interceptor events continue to use the existing per-capsule queue fast path.

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A byte is sent with care, Through chains of filters there. Deny, or Final say, To change the flow today, And alter fate's affair.

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[gemini-code-assist]

Code Review

This pull request introduces a valuable short-circuiting mechanism for the interceptor chain, allowing interceptors to return Continue, Final, or Deny to control the execution flow. The implementation is well-structured, with a clear InterceptResult enum and backward-compatible wire format handling. The dispatcher logic is updated to manage the chain, including a fast-path for single-interceptor events, and the new functionality is thoroughly covered by tests.

I've found one issue in the single-interceptor dispatch logic where the interceptor's result was previously ignored. My review includes a suggestion to address this, ensuring proper handling of Result outcomes and preventing silent failures, consistent with repository guidelines.