feat(cache): restore task results from the remote cache

[wan9chi]

Motivation

#755 uploads task results to the remote cache, but nothing reads them back. Restoring them lets a task that ran on another machine, or before vp cache clean, replay instead of running again.

Changes

• vt_remote_cache adds fetch (POST {endpoint}/fetch), which returns an exact match, a fallback match with its key, or no match, and download (GET {endpoint}/blob/{blob_id}), which streams the blob's chunks.
• After a local miss in read or read-write mode, vp run fetches the entry. An exact entry is validated like a local one. Its archive is downloaded to a .tmp file while being checked from the same stream, then renamed, recorded locally, and replayed as a hit. Hits never upload.
• A fallback, a failed validation, or a failed read is a miss. If the local cache had an entry, its miss reason stays; otherwise the reason comes from the remote cache, such as remote cache fetch failed or downloaded archive is corrupt. Read failures aren't warnings, and --verbose and --last-details show their causes like other errors (refactor(reporter): save errors as a chain of causes #761). After a failed read, the .tmp file is removed and nothing is recorded.

[github-actions]

fspy benchmark

linux

dynamic/launch             change  -0.68%  [-12.89% .. +16.60%]  overhead  +295.50%
dynamic/access             change  -0.42%  [ -5.38% ..  +7.19%]  overhead   +23.02%
dynamic/access-relative    change  +0.30%  [ -2.40% ..  +3.66%]  overhead   +73.69%
dynamic/access-contended   change  -2.99%  [-13.39% ..  +7.78%]  overhead   +52.31%
static/launch              change  +1.54%  [ -7.66% .. +13.17%]  overhead  +706.43%
static/access              change  -0.23%  [ -2.03% ..  +1.71%]  overhead +1472.91%
static/access-relative     change  -0.34%  [ -2.34% ..  +1.90%]  overhead +1915.78%
static/access-contended    change  -0.80%  [ -1.62% ..  +1.78%]  overhead +1833.17%

macos

dynamic/launch             change  +0.09%  [ -5.04% ..  +6.50%]  overhead  +243.59%
dynamic/access             change  +0.34%  [-25.95% .. +104.52%]  overhead    +5.32%
dynamic/access-relative    change  -0.61%  [-18.95% ..  +3.67%]  overhead  +261.70%
dynamic/access-contended   change  +2.02%  [-27.33% .. +24.98%]  overhead    +4.72%

windows

dynamic/launch             change  -0.89%  [ -7.60% ..  +7.13%]  overhead   +26.11%
dynamic/access             change  +0.00%  [ -1.82% ..  +2.43%]  overhead    +1.30%
dynamic/access-relative    change  +0.18%  [ -1.27% ..  +2.00%]  overhead    +1.30%
dynamic/access-contended   change  -0.22%  [ -7.45% ..  +4.03%]  overhead    +1.23%