feat(cache): upload to the remote cache in the background and support HTTP/2

[wan9chi]

Motivation

In read-write mode, a task waited for its upload to the remote cache before it counted as finished, so every task that depended on it waited too. A slow remote cache slowed down the whole run, even though nothing in the run needed the upload. Once uploads run side by side, HTTP/1.1 also opens a connection for each one; HTTP/2 lets them share one.

Changes

• Background uploads. Once a task's result is cached locally, its upload starts in the background and the task finishes right away. When the graph is done, vp run prints Waiting for N remote cache uploads to finish (Ctrl-C to cancel)... and waits for them before the summary. An upload's error is set later, through the Arc<OnceLock<UploadError>> in CacheUpdateStatus::Updated, and the summary reads it after the wait.
• Cancellation. A new interrupt token, which only Ctrl-C cancels, cancels the uploads. Their entries stay in the local cache, and the summary says they weren't uploaded because they were interrupted. Fast-fail still stops lookups but no longer stops uploads, so tasks that succeeded before another one failed are still uploaded. docs/cancellation.md covers both.
• HTTP/2. reqwest's http2 feature is on. HTTPS endpoints use HTTP/2 if the server accepts it in the TLS handshake and HTTP/1.1 otherwise. http:// endpoints stay on HTTP/1.1.
• Tests. Whether an upload is still running when the graph is done depends on how fast the remote cache responds. So e2e steps that upload set VP_RUN_INTERNAL_HIDE_PENDING_UPLOADS, which keeps the waiting line out of their output, including the first step of restore_failure from fix(cache): fail the run when a cache hit's outputs can't be restored #770. remote-cache-server takes --stall ROUTE, which reads requests to the route but never answers them, so the cases that show the waiting line or cancel uploads use --stall /store. Like the other backend cases, they need Node.js and are skipped on Windows. New e2e cases: pending_uploads, hide_pending_uploads, ctrl_c_during_upload, and fast_fail_during_upload. A unit test checks that the client offers h2 over TLS.

Notes for reviewers

• Uploads aren't capped. Each upload in progress holds its encoded entry in memory and its output archive open, plus its own connection on HTTP/1.1, and none of that counts against --concurrency-limit. A large graph with a slow endpoint can pile them up. A cap is left for a follow-up.
• No HTTP/3. reqwest's http3 feature needs --cfg reqwest_unstable in every build, vite-plus included, adds aws-lc-rs next to ring, bypasses proxies, and is used only when the client forces HTTP/3 for every request.

🤖 Generated with Claude Code

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +0.38%  [ -5.66% ..  +6.57%]  overhead  +296.59%
dynamic/access             change  -0.29%  [ -2.38% ..  +1.21%]  overhead   +13.25%
dynamic/access-relative    change  +0.56%  [ -1.63% ..  +1.65%]  overhead   +60.00%
dynamic/access-contended   change  +3.37%  [-10.10% .. +22.50%]  overhead   +22.03%
static/launch              change  +0.41%  [ -5.60% ..  +5.70%]  overhead  +730.87%
static/access              change  +0.16%  [ -7.20% ..  +7.71%]  overhead  +821.90%
static/access-relative     change  -0.11%  [ -6.59% ..  +4.79%]  overhead +1308.93%
static/access-contended    change  +3.35%  [ -6.12% .. +13.27%]  overhead +2234.33%

macos

dynamic/launch             change  -0.07%  [ -4.82% ..  +4.95%]  overhead  +238.74%
dynamic/access             change  +1.36%  [ -9.74% .. +27.72%]  overhead    +0.94%
dynamic/access-relative    change  -0.56%  [-11.34% .. +62.71%]  overhead  +251.63%
dynamic/access-contended   change  +5.32%  [ -7.69% .. +628.75%]  overhead    +7.23%

windows

dynamic/launch             change  +0.41%  [-13.04% .. +18.02%]  overhead   +27.90%
dynamic/access             change  -1.07%  [-20.43% ..  +7.71%]  overhead    +3.32%
dynamic/access-relative    change  -2.65%  [-16.46% .. +13.96%]  overhead    +0.00%
dynamic/access-contended   change  +3.09%  [ -6.82% .. +14.94%]  overhead    +5.45%