# Session HTTP Routes Landed Locally

## Context
M4e needed session-scoped HTTP save/load plumbing on top of the existing server engine, guarded by the same bearer API key path as the rest of `/v1`, plus a local integration test seam that does not depend on a real CUDA scheduler. The work also needed to preserve non-CUDA and Metal builds while exposing just enough engine hooks for snapshot save/load.

## What Worked
Adding defaulted session methods to `InferenceEngine` let the HTTP layer compile against existing engines without forcing every backend to implement the new surface immediately. Keeping the session HTTP router on `/save`, `/load`, `/manifest`, and `/` and then wrapping it at `/{session_id}` before mounting under `/v1/sessions` preserved the requested external route shape while avoiding state-type leakage into the main app router. A mock engine backed by `DiskStore` and `RadixCache` gave local end-to-end coverage for snapshot success, format mismatch, tamper detection, and body-limit enforcement. The CUDA byte-copy helpers on `PagedKVPool` are deliberately left as `todo!()` in the CUDA branch and `Err(...)` in the no-CUDA branch so the API shape is in place without pretending the host-copy path is validated on this Mac.

## Rule
When a new HTTP feature depends on scheduler internals, add the smallest defaulted engine surface first, prove the route contract with a mock engine locally, and leave CUDA-only byte movement behind an explicit `todo!()` until the next L4 validation window on real hardware.
