docs: document request body streaming and upstream HTTP/2

[slin1237]

v1.10.0 reworked how the HTTP router handles request bodies and upstream connections.

• Large request bodies stream to the worker instead of being buffered (feat(router): stream large request bodies past a size threshold smg#2198).
• The router now decides stream-vs-buffer per request (#2286). The --stream-request-bodies-over flag from pre-release builds was removed before v1.10.0 shipped.
• A stalled upload is aborted with 408 (#2201). Request buffers are freed at dispatch (#2232, #2237), and worker responses are capped (#2138).
• HTTP/2 to workers is opt-in and negotiated per worker (#2155, #2407). Pooled connections expire before the backend's keep-alive, and a send that fails before any response is resent once (#2162).

None of this was documented. The retries page also had several errors:

• It miscounted attempts. --retry-max-retries includes the first attempt.
• It listed a backoff delay that the defaults never reach.
• It described half-open circuits as "limited test requests".
• It had no word on streamed bodies or on responses that are never retried.

Changes

• New concept page, concepts/performance/request-streaming.md ("Request Streaming and Upstream Connections"):
  • Which routes and router modes can stream.
  • The ordered stream-vs-buffer rules, each with its reason metric label.
  • What changes for a streamed request: no router retries, and validation happens at the worker.
  • The 408/413/400 upload aborts.
  • Memory bounds and response relaying: backpressure, cancel on disconnect, empty h2 frames, TCP_NODELAY.
  • Connection pools and http_pool overrides, and the one-shot resend.
  • Upstream HTTP/2: negotiation, what traffic uses it, pinning, checking, and mixed-fleet rollout.
  • Configuration and metrics.
• concepts/reliability/retries.md:
  • How a retry works: attempts include the first, and selection is re-run.
  • Where retries apply, by router and endpoint.
  • A corrected backoff table with jitter ranges.
  • A new "Never Retried" section: overload sheds, tenant denials, and admission rejections.
  • Retries and request bodies, and the pre-response resend.
  • A rewritten circuit-breaker interaction, with capacity pushback retried but not counted.
  • Metrics labels and PromQL fixes.
  • The per-worker resilience retry fields are documented as having no effect in v1.11.0. See docs(api): document the full worker spec for the worker management API #43: registration never stores them on the worker, so no per-model retry policy is created.
• Nav and concepts index: the page is listed under Performance.

Verification

• Checked against smg v1.11.0 (3be823a7):
  • routers/common/body_policy.rs, routers/http/{router,request_stream,request_body}.rs, routers/common/request_lease.rs
  • worker/http_client.rs, workflow/steps/local/detect_connection.rs
  • crates/external_router/src/{retry,retry_config,sse,header_utils}.rs, workflow/steps/shared/register.rs
  • main.rs and observability/metrics.rs
• pnpm lint, pnpm check (0 errors), and pnpm build pass.
• The link check shows only pages added by sibling PRs: sticky-sessions.md (docs(routing): document sticky sessions and routing headers #42) and overload-protection.md (docs(reliability): document worker overload protection and load monitoring #37).
• All three pages render with status 200 under wrangler pages dev.

This is one of a series of docs PRs that bring the site up to smg v1.11.0, one per area.

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📒 Files selected for processing (4)

• src/lib/config/concepts-nav.ts
• src/lib/content/concepts/index.md
• src/lib/content/concepts/performance/request-streaming.md
• src/lib/content/concepts/reliability/retries.md

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