test(cache): run commands against the remote cache test server

[wan9chi]

Motivation

The remote cache client PRs stacked on this one need to run vp against the test backend, with uploads from one step visible to the next, for example to restore outputs after vp cache clean. The detached daemon needs explicit start and stop steps and loses its state on restart. Its fetch responses also differ from the server API in #713, which the client will implement.

remote-cache-server <command> [args...] starts the backend on a free loopback port under the /projects/test base path, runs the command with VP_REMOTE_CACHE_URL set to that endpoint, and exits with the command's exit code after stopping the server. The command inherits stdio, and the wrapper takes no options. State lives in remote-cache/ in the working directory: state.json holds entries, associations, and the next blob ID, and each blob is a file in blobs/ named by its ID. Consecutive steps share state, and blob IDs keep counting across invocations, so snapshots stay deterministic.

This removes start, stop, the daemon, cache.url, cache.log, the --base-path, --max-request-bytes, and --max-lifetime-ms options, and the request size limit. cbor-http joins its path onto VP_REMOTE_CACHE_URL, including the base path. As in #713, a fetch miss returns 200 {kind: "not_found"}, and a fallback returns {kind: "fallback", key, value, blob_id}.

The remote_cache_backend steps now run as remote-cache-server cbor-http .... The request_limit case is removed. daemon_lifecycle is replaced by state_across_invocations, and a new command_wrapper case covers the endpoint's base path and the exit code.

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +0.67%  [ -7.15% ..  +8.61%]  overhead  +288.03%
dynamic/access             change  -0.42%  [ -6.01% ..  +1.94%]  overhead   +12.95%
dynamic/access-relative    change  +0.13%  [ -8.50% .. +21.90%]  overhead   +62.97%
dynamic/access-contended   change  +3.70%  [ -6.68% .. +15.94%]  overhead   +15.45%
static/launch              change  +1.42%  [ -4.79% ..  +8.80%]  overhead  +690.81%
static/access              change  -0.38%  [-11.39% ..  +9.78%]  overhead  +837.60%
static/access-relative     change  +1.09%  [ -4.94% ..  +5.72%]  overhead +1244.27%
static/access-contended    change  +0.10%  [ -1.21% ..  +1.19%]  overhead +3157.87%

macos

dynamic/launch             change  +0.64%  [ -2.22% ..  +3.23%]  overhead  +211.51%
dynamic/access             change  -0.37%  [ -4.51% ..  +5.42%]  overhead    +8.82%
dynamic/access-relative    change  -0.34%  [ -2.22% ..  +2.28%]  overhead  +246.43%
dynamic/access-contended   change  +0.80%  [ -1.28% ..  +3.45%]  overhead    +4.26%

windows

dynamic/launch             change  -0.08%  [ -8.88% ..  +7.37%]  overhead   +24.99%
dynamic/access             change  -0.35%  [ -2.97% ..  +4.69%]  overhead    +1.40%
dynamic/access-relative    change  -0.17%  [-12.42% .. +13.06%]  overhead    +2.55%
dynamic/access-contended   change  +1.39%  [ -3.87% ..  +4.84%]  overhead    +3.47%