[DavidBellamy]

Description

Problem

The bounded K3 distribution-seed path excluded streaming requests because its success hook only ran after non-streaming response processing. Most live K3 traffic is streaming, so hot cache owners could retain localized queues while eligible peers remained idle.

Request shape alone is not a safe authorization boundary: the Responses API internally executes chat-shaped streams, and Harmony also uses a chat request type.

Solution

Allow only direct Regular Chat Completions scalar streams to use distribution seeding. A private producer witness records backend completion, successful stream processing, and successful terminal [DONE] enqueue. The response body retains the existing non-cloneable load guards and commits the provisional owner only after clean client-visible EOF. Disconnects, body errors, missing backend completion, processing errors, and send failures remain uncommitted and enter the existing failure quarantine.

A typed invocation plus authoritative pipeline endpoint and mode excludes internal Responses iterations, Harmony, PD/EPD, fanout, and other streaming endpoints. The partition seed cap remains one.

Changes

• Admit direct Regular Chat Completions scalar streams to the existing distribution-seed path.
• Add a two-phase producer/body success witness for terminal seed commit.
• Preserve fail-closed quarantine and exact lease release on every incomplete stream path.
• Add trusted pipeline-origin discrimination for direct Chat versus internal Responses calls.
• Add lifecycle, endpoint-scope, fanout, and cap-one regressions.

Closes #65

Test Plan

• cargo test -p smg --lib: 1,551 passed, 0 failed, 5 ignored.
• Endpoint derivation matrix: 1 passed.
• Adaptive streaming eligibility: 5 passed.
• Streaming producer/body lifecycle: 5 passed.
• Distribution-seed regressions: 10 passed.
• Distribution-headroom and cap-one regressions: 12 passed.
• cargo +nightly fmt --all -- --check: passed.
• git diff --check: passed.
• Strict SMG library Clippy with -D warnings: passed with only the two documented, unchanged base-lint allowances. The local all-features gate remains unavailable because the OpenCV system dependency is not installed.

Checklist

• cargo +nightly fmt passes
• cargo clippy --all-targets --all-features -- -D warnings passes locally (blocked by missing system OpenCV; strict changed-library Clippy passes)
• Documentation updated (not applicable; no public configuration or API change)