fix(background-terminals): extend Windows taskkill force window

[outlier27-cell]

Problem

Fixes #615. On Windows, the background-terminal shutdown preserves a 2.5s total budget but allocates only 500ms to the force taskkill /T /F phase (400ms before its helper-close reserve). On this host, the force helper regularly needs longer, so a real descendant-tree shutdown can become fail-closed unresolved even though the helper would complete within the existing overall bound.

This does not reopen #408/#412: a helper timeout still remains unresolved and never falls back to shell-only termination.

Value

Gives Windows process-tree termination enough bounded time to complete on slower taskkill hosts while retaining the existing lifecycle deadline and fail-closed behavior.

Approach

• Allocate the existing 2.5s Windows termination budget as 1500ms graceful plus 1000ms force.
• Keep the 2000ms plus 500ms allocation unchanged on non-Windows platforms.
• Do not change taskkill timeout semantics, direct fallback policy, ownership, or process-tree confirmation rules.

Validation

• Fresh worktree from origin/main 43cd2e94cc584f81cc1f27512ae99c9a4099b231.
• RED: isolated real Windows descendant-tree regression failed on unchanged main in about 4.95s.
• GREEN: same isolated regression passed after the change in about 3.57s.
• GREEN: isolated kill settles a never-exiting process passed in about 3.28s.
• GREEN: 12 Windows taskkill contract tests passed.
• GREEN: bun run check passed.
• bun run test was started; it remained running after the command observer returned. This repository has an existing post-edit real-Pi timeout/hang cluster, so this PR does not claim a full-suite pass. A combined real-process pressure filter also still made the descendant assertion timing-sensitive; this is recorded in fix(background-terminals): give Windows taskkill /T /F enough bounded time #615 rather than hidden.

Impact

• User-visible behavior: fewer avoidable Windows background-terminal termination failures on slower taskkill hosts.
• Model-visible context/tools: none.
• Runtime/lifecycle: Windows phase allocation changes only; total bounded termination window and fail-closed timeout outcome are unchanged.
• Persisted config/data: none.
• Compatibility/risk: Windows timing is host dependent; this is not Job Object containment and does not make an uncertain tree kill appear successful.