test(cache): emit the stalled milestone only when a test waits for it

[wan9chi]

Motivation

remote_cache::ctrl_c_during_upload passed on #841 but timed out on main in Test (windows-ignored), waiting for the uploads-pending milestone.

On Windows a milestone is the console title, and ConPTY sends only the latest title when it next renders. vtt stalled-remote-cache emits a stalled milestone when the upload arrives, which can be just after vt run emits uploads-pending. stalled then replaces it, and the harness waits until the test times out. fast_fail_during_upload has the same race, and it's why pending_uploads and hide_pending_uploads timed out on Windows in #841's first CI run. Only ctrl_c_during_fetch waits for stalled.

Changes

• vtt stalled-remote-cache emits the stalled milestone only with the new --milestone flag, which ctrl_c_during_fetch passes.
• pending_uploads and hide_pending_uploads run on Windows again. test(cache): run the remote cache e2e tests on Windows #841 skipped hide_pending_uploads because nothing is printed after uploads-pending, but ConPTY sends a title change without other output, as milestone_raw_mode_keystrokes and vtt exit-on-ctrlc show.

🤖 Generated with Claude Code

[github-actions]

fspy benchmark

linux

dynamic/launch             change  -0.91%  [ -9.23% ..  +7.66%]  overhead   +48.72%
dynamic/access             change  +0.61%  [-17.57% .. +25.44%]  overhead    +9.79%
dynamic/access-relative    change  +0.44%  [-22.93% .. +27.12%]  overhead   +59.14%
dynamic/access-contended   change  -0.21%  [ -8.67% .. +12.39%]  overhead    +8.49%
static/launch              change  +0.80%  [ -9.46% .. +10.82%]  overhead  +156.38%
static/access              change  -0.42%  [-10.84% ..  +7.78%]  overhead  +887.33%
static/access-relative     change  +0.88%  [ -4.22% ..  +4.51%]  overhead +1201.58%
static/access-contended    change  +2.64%  [ -8.91% ..  +9.13%]  overhead +1876.66%

macos

dynamic/launch             change  +0.30%  [ -4.43% ..  +4.05%]  overhead  +229.69%
dynamic/access             change  +0.54%  [-20.61% .. +140.27%]  overhead    +5.04%
dynamic/access-relative    change  +0.32%  [ -5.29% ..  +5.62%]  overhead  +263.17%
dynamic/access-contended   change  -0.63%  [-16.99% .. +24.78%]  overhead    +5.86%

windows

dynamic/launch             change  -0.51%  [ -4.11% ..  +2.80%]  overhead   +25.22%
dynamic/access             change  -0.18%  [ -1.06% ..  +0.73%]  overhead    +1.25%
dynamic/access-relative    change  +0.00%  [ -0.72% ..  +1.25%]  overhead    +1.63%
dynamic/access-contended   change  +0.53%  [ -0.87% .. +14.76%]  overhead    +1.23%