Windows: normalize runtime environments and portable tests

[Yuzu02]

Layer 2: Windows portability and QA

Depends on #1 and contains only the portability/static-analysis layer:

• normalize Windows command-environment keys case-insensitively
• preserve canonical Path, ComSpec, SystemRoot, and related names
• supply PATHEXT for intentionally minimal command environments
• replace Bash-only test commands with equivalent PowerShell/Python fixtures on Windows
• preserve exact LF fixtures without changing lossless production read semantics
• guarantee runtime cleanup when assertions fail
• make telemetry home and POSIX permission assertions platform-aware
• resolve the remaining Mypy issues in process, transport, result, and server helpers

Validation on the stack tip:

• 217 tests passed
• 0 failures, 0 errors
• 87 explicit POSIX/Linux-only skips
• Ruff clean
• Mypy clean across 16 source files
• uv lock --check clean

Upstream contribution starts with xyTom#37. GitHub's public-preview stacks cannot cross forks, so this layer will be retargeted upstream after layer 1 lands or if the maintainer adopts the branches into upstream.