docs: Actor App-Initiated gRPC Event Streams (SubscribeActorEventsAlpha1)

[nelson-parente]

Summary

Documents SubscribeActorEventsAlpha1, the flagship new alpha API in Dapr v1.18 that lets actor-hosting apps receive all callbacks over a single bidirectional gRPC stream initiated from the app to the sidecar, with no inbound server port required.

• Closes the 5-year-old feature request Implement actor APIs for gRPC dapr#927 ("Implement actor APIs for gRPC")
• Implemented in Actors: app-initiated gRPC stream for actor callbacks dapr#9812 (merged, backport #9964)
• Targets the v1.18 branch

Files changed

File	Description
daprdocs/content/en/developing-applications/building-blocks/actors/actors-app-initiated-streams.md	New concept page. Explains the app→sidecar streaming protocol, why it exists (replaces inbound actor callbacks), comparison table with traditional callbacks, protocol walk-through, rolling-restart / multi-connection behavior, and NetworkPolicy / firewall guidance for operators including a sample Kubernetes NetworkPolicy manifest. Marked Alpha prominently.
daprdocs/content/en/developing-applications/building-blocks/actors/howto-actors-app-initiated-streams.md	New how-to page. Step-by-step guide: open the stream, send the initial registration, handle all four callback types (invoke, reminder, timer, deactivate), signal errors with request_failed, and implement reconnection with exponential back-off. Includes full raw gRPC Go code examples.
daprdocs/content/en/reference/api/actors_api.md	Extended. New SubscribeActorEventsAlpha1 (gRPC) section with the gRPC service definition and complete field-level reference tables for all request/response proto messages.
daprdocs/content/en/operations/support/alpha-beta-apis.md	Updated. Added Actor App-Initiated Streams row to the Alpha APIs table.

Test plan

• Verify Hugo build passes (no broken {{< ref >}} links)
• Confirm weight: 55 and weight: 56 don't collide with existing actor pages (current highest non-how-to weight in the folder is actors-timers-reminders.md — check _index.md ordering)
• Cross-check proto field names against merged dapr/dapr#9812 once final proto is tagged in v1.18
• Confirm SDK-specific code examples can be added once .NET / Python / Java SDK PRs land (tracked separately)
• Verify NetworkPolicy example port matches the actual default daprd gRPC port annotation default

Open questions / TODOs for reviewer

1. SDK examples: The how-to currently has only a raw Go gRPC example. SDK-level wrappers for .NET, Python, Java, and Go are being added in parallel SDK PRs. Once those land, SDK tabs should be added to the {{< tabpane >}} in the how-to.
2. Proto link in alpha-beta-apis.md: The link to the proto file points to master. Replace with a tagged v1.18 permalink once the release tag is cut.
3. Weight values: weight: 55 and weight: 56 are slotted between howto-actors.md (no explicit weight) and the existing pages. Verify the rendered nav order looks right.
4. howto-actors-partitioning.md weight: The v1.18 branch may have different weight values if other actor pages were added. Double-check.
5. actors-overview.md cross-link: Consider adding a bullet under the "Features" section of actors-overview.md pointing to the new concept page for discoverability. Not blocking.

Targets v1.18. Draft pending review.