docs: map the two admission-lane systems for operators (#9654)

U5 from KC plan 2026-08-09-001. Verifies lane metrics are exposed by the health
payload (GET /api/monitoring/health -> adaptiveAdmission -> lane* fields) and
records which lane system reports where: byte-level per-connection lanes (always
on, memory scope) vs adaptive virtual lanes (opt-in via OMNIROUTE_CHAT_VIRTUAL_LANES,
dispatch scope) plus the explicit opt-in ops note.

docs/architecture/admission-lanes.md

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+# Admission lanes (#9654) — two lane systems, what gates each, where each reports
+
+OmniRoute has **two** process-local lane systems with different scopes. They are
+complementary; operators should know which one they are looking at.
+
+## 1. Byte-level per-connection lanes (`chatBodyAdmission.ts`)
+
+- **Scope:** the buffered-body/heap path for `POST /v1/chat/completions`. Guards
+  against heap amplification from large coding-agent bodies (#4380).
+- **Gate:** **always on.** Each distinct API key (hashed) — or `anonymous` — gets its
+  own lane with `CHAT_MAX_HEAVY_IN_FLIGHT` capacity, so one session's burst cannot
+  starve another session's heavyweight slot.
+- **Tuning:**
+  - `OMNIROUTE_CHAT_VIRTUAL_TTL_MS` — idle-lane eviction (default 60000)
+  - `OMNIROUTE_CHAT_VIRTUAL_MAX_SESSIONS` — lane count cap (default 64)
+  - `OMNIROUTE_CHAT_ADMISSION_QUEUE_MS` — queue-wait before 503 (default 2000)
+  - `OMNIROUTE_CHAT_ADMISSION_MAX_QUEUED_BYTES` — queued-bytes heap valve (default 4 MB)
+- **Reports:** not in `GET /api/monitoring/health` today; observable via
+  `PerConnectionAdmissionController.snapshot()` (sessionId hash, activeHeavy, idleMs).
+
+## 2. Adaptive runtime virtual lanes (`open-sse/services/admission`)
+
+- **Scope:** tenant-key admission for provider dispatch — queue cost, latency-guided
+  limit adaptation, lane queueing, and lane metrics.
+- **Gate:** **opt-in.** Disabled unless `OMNIROUTE_CHAT_VIRTUAL_LANES=true`. Without it,
+  the adaptive controller keeps the shared queue behavior (criterion 1 of #9654 only
+  holds once an operator enables lanes).
+- **Tuning:** `OMNIROUTE_CHAT_VIRTUAL_LANES` + adaptive config (`maxQueueCount`,
+  `maxQueueCost`, `defaultMaxWaitMs`, …).
+- **Reports:** `GET /api/monitoring/health` → `adaptiveAdmission` → `laneCount`,
+  `laneQueuedCount`, `laneQueuedCost`, `laneTenants` (opaque lane IDs, never raw keys).
+
+## Which one is showing in a dashboard
+
+- `adaptiveAdmission.laneCount` / `laneTenants` → **adaptive virtual lanes** (system 2).
+- A health payload with **no** `adaptiveAdmission.lane*` fields usually means
+  `OMNIROUTE_CHAT_VIRTUAL_LANES` is unset — the byte-level lanes (system 1) are still
+  active, but nothing under `adaptiveAdmission` will report lane data until it is enabled.
+
+## Why both exist
+
+The byte-level lanes bound the memory-heavy parse/compress path; the adaptive lanes
+bound dispatch cost per tenant. #9654's criterion 1 ("one session's burst does not 503
+another") is enforced by system 1 unconditionally and by system 2 once opt-in is enabled.