test: wait for each Windows milestone to be seen before the next

[wan9chi]

Motivation

On Windows a milestone is the console title, and ConPTY emits only the latest title when it renders. Another process's milestone could replace a milestone before that render. In pending_uploads and hide_pending_uploads, the proxy's stalled milestone can replace uploads-pending. Both cases timed out waiting for it in Windows CI, so #841 skipped them on Windows.

A side channel alone can't replace the title. The harness must also know that the screen includes everything printed before the milestone, and ConPTY guarantees that only for its rendered output (see #516). So the title stays, but each producer waits until the harness has seen its title.

Changes

• On Windows, mark_milestone takes a named lock shared by the processes in the test terminal, sets its title, and holds the lock until the harness signals an event named after the milestone's random id. TestTerminal::spawn passes the lock's name in VT_PTY_TERMINAL_TEST_MILESTONE_LOCK. A process that doesn't inherit this variable, such as a task that vp runs with a filtered environment, only sets the title, as before.
• Reader signals the event for each milestone it reads, in both expect_milestone and wait_for_exit. decode_milestone_title returns the id along with the name.
• New vt_pty_terminal_test tests: two processes marking milestones at once, and a milestone with nothing printed after it.
• pending_uploads and hide_pending_uploads run on Windows again.

🤖 Generated with Claude Code

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +0.50%  [ -8.69% ..  +8.61%]  overhead   +50.80%
dynamic/access             change  +0.30%  [-18.35% .. +25.09%]  overhead    +7.35%
dynamic/access-relative    change  +1.48%  [-10.36% .. +12.38%]  overhead   +50.19%
dynamic/access-contended   change  +2.01%  [ -2.20% ..  +7.71%]  overhead   +13.63%
static/launch              change  +1.35%  [ -7.59% .. +10.41%]  overhead  +157.79%
static/access              change  -2.41%  [-14.41% .. +10.79%]  overhead  +746.23%
static/access-relative     change  -1.96%  [ -9.04% ..  +7.62%]  overhead  +995.60%
static/access-contended    change  -0.05%  [ -9.47% ..  +6.64%]  overhead +2004.30%

macos

dynamic/launch             change  -0.08%  [ -4.57% ..  +4.59%]  overhead  +235.06%
dynamic/access             change  +0.67%  [ -4.37% .. +13.40%]  overhead    +5.22%
dynamic/access-relative    change  -0.60%  [ -5.16% ..  +6.11%]  overhead  +239.08%
dynamic/access-contended   change  -1.18%  [-13.17% .. +32.33%]  overhead    +3.76%

windows

dynamic/launch             change  +0.65%  [ -4.36% ..  +6.39%]  overhead   +26.28%
dynamic/access             change  +0.19%  [ -1.33% ..  +0.96%]  overhead    +1.35%
dynamic/access-relative    change  +0.00%  [ -0.93% ..  +1.13%]  overhead    +1.35%
dynamic/access-contended   change  -0.36%  [ -2.52% ..  +1.10%]  overhead    +1.99%