fix(cache): stop remote cache requests on Ctrl-C and fast-fail

[wan9chi]

Motivation

Remote cache requests ignored cancellation. After Ctrl-C or a failing task, vp run still waited for in-flight lookups, downloads, and uploads to finish or hit the 60s read timeout, and a task whose lookup finished after cancellation could still start.

Changes

• Ctrl-C and fast-fail now share one cancel token: the Ctrl-C token is a child of the fast-fail token, so every fast-fail (including a pipe failure in run_child) cancels it too, while Ctrl-C still leaves running processes alone.
• Remote fetches, archive downloads, and uploads race against that token. A cancelled download still waits for the archive check to close the .tmp file before removing it.
• A task that's cancelled during its cache lookup doesn't start and doesn't restore cached outputs. Like a task that was never scheduled, it isn't shown, and it doesn't trigger fast-fail.
• A cancelled upload keeps the local entry, and the summary shows the existing warning with cancelled as the reason.
• Tests: unit tests for cancelling a fetch, an upload, and a download mid-body (the .tmp file is removed); e2e cases for Ctrl-C and fast-fail during a fetch, using a new vtt stalled-remote-cache helper that serves an endpoint that never responds. Neither needs Node, so they run on all platforms.
• Updated docs/cancellation.md and the remote caching changelog entry.

Closes #766

[github-actions]

fspy benchmark

linux

dynamic/launch             change  -0.54%  [ -7.04% ..  +8.25%]  overhead  +268.20%
dynamic/access             change  +0.31%  [ -0.76% ..  +1.25%]  overhead   +13.18%
dynamic/access-relative    change  -0.23%  [ -0.87% ..  +0.99%]  overhead   +60.40%
dynamic/access-contended   change  -0.28%  [ -2.23% ..  +3.05%]  overhead   +14.88%
static/launch              change  +0.48%  [ -5.45% ..  +7.27%]  overhead  +734.29%
static/access              change  +0.19%  [ -0.69% ..  +1.10%]  overhead  +807.13%
static/access-relative     change  +0.11%  [ -0.93% ..  +0.96%]  overhead +1395.66%
static/access-contended    change  +0.16%  [ -0.83% ..  +1.33%]  overhead +3140.92%

macos

dynamic/launch             change  -0.39%  [ -4.65% ..  +4.59%]  overhead  +242.73%
dynamic/access             change  -1.20%  [-28.10% .. +15.08%]  overhead    +4.28%
dynamic/access-relative    change  +3.27%  [ -7.84% .. +80.23%]  overhead  +283.10%
dynamic/access-contended   change  -0.52%  [-87.00% ..  +7.24%]  overhead    +2.18%

windows

dynamic/launch             change  +0.60%  [ -5.23% ..  +6.94%]  overhead   +25.72%
dynamic/access             change  -0.37%  [-20.00% ..  +7.91%]  overhead    +1.12%
dynamic/access-relative    change  +0.00%  [-19.26% ..  +8.50%]  overhead    +2.83%
dynamic/access-contended   change  -0.51%  [ -9.55% ..  +7.43%]  overhead    +5.02%