Correct README verification claims

[Akotz89]

Summary

• replace stale shellfix_*.ps1 verification guidance with mode-specific checks
• document one-shot, session proxy, and -File fallback verification separately
• align native tool claims with the profile-layer implementation
• document the pre-built release asset layout and checksum flow
• add explicit known limitations for profile loading, WSL distro availability, native wrapper allowlist, and IDE PATH inheritance
• update test counts to match the current suites

Fixes

• Fixes Correct README verification and behavior claims #14

Verification

• .\test-ci-smoke.ps1 -ShimPath .\shim\out\powershell.exe -WslDistro Ubuntu-24.04 -> 5/5 pass
• .\test.ps1 -ShimPath .\shim\out\powershell.exe -WslDistro Ubuntu-24.04 -> 48/48 pass
• .\test-proxy.ps1 -ShimPath .\shim\out\powershell.exe -WslDistro Ubuntu-24.04 -> 16/16 pass
• .\test-replay.ps1 -ShimPath .\shim\out\powershell.exe -WslDistro Ubuntu-24.04 -> 10/10 pass
• Verified -File fallback debug output includes Dangerous quoting detected and shellfix_.ps1
• git diff --check

Risk

Docs-only. No runtime behavior changed.