fix(stargate): address protocol, lifecycle, recovery, and benchmark audit findings

[barrygreengus]

Problem

A Stargate maintenance review identified correctness and resource-lifetime defects in protocol handling, proxy retries, registration recovery, and benchmark reporting.

Resolution

Implemented in a four-layer draft PR stack:

Domain	Corrections	PR
Protocol contracts	IPv6 backend parsing, owned and escaped JSON inputs, connection-specific HTTP headers, and direct schema generation with behavior tests	#1818
Proxy and relay behavior	Prevent duplicate accepted POST retries, reap completed relay tasks, and bound unknown-target metrics	#1820
Registration recovery	Kubernetes watcher backoff, non-TLS warnings on every failed retry, safe diagnostics, and token-file redaction	#1823
Benchmark accuracy	Validate completed SSE output, report observed token usage, preserve unknown measurements, and run scenario tests through Bazel	#1826

The router deployment uses raw QUIC. Bidirectional-stream priority is intentional, and the proposed stream-fairness correction has been removed from #1820.

The output-timeout defect was already fixed by #1452 on main. Its first-output and subsequent-output deadline regressions were verified as part of the workspace tests.

Verification

• Bidirectional-priority restoration: all 24 stargate-forwarding tests passed; all-target Clippy passed with warnings denied. Removed the fair-direction helper, enum, and backlog fixture.
• Direct tracing update: 515 pylon-lib unit tests and 2 public API tests passed; all-target pylon-lib Clippy passed with warnings denied. Retry warnings now use tracing macros at the failure sites; safe formatting remains separate.
• Prior combined stack validation: 1,611 Cargo workspace tests passed, 3 existing performance tests ignored, workspace Clippy passed, and 6 scoped Bazel targets passed.
• Changed Rust files passed formatting checks; the stack passed whitespace and fanout validation.

No GPU or live-cluster performance campaign was run. The PRs are draft and have not been merged.