feat: native Windows plugin pane spawning with backward compatibility

[Only-Moon]

Summary

This PR adds native Windows plugin pane spawning support (available since herdr v0.8.0-preview) with backward compatibility for older herdr versions.

Changes

Native Windows Pane Spawning

• Cross-platform pane entrypoint: command = ["herdr-file-viewer"] with platforms = ["linux", "macos", "windows"]
• Build scripts create PATH shim at ~/.local/bin/herdr-file-viewer.exe (symlink to binary)
• Native pane spawning since herdr v0.8.0 (commit d3be4f8b) - direct CreateProcessW spawn, no shell

Windows Launcher Scripts (with version detection & fallback)

• scripts/open-file-viewer-windows.ps1 - split pane
• scripts/open-file-viewer-tab-windows.ps1 - new tab
• Version detection: checks herdr --version, uses native if >= v0.8.0
• Fallback: pane split + pane run + send-keys for herdr < v0.8.0

Build System

• Updated scripts/fetch-or-build.ps1 and scripts/fetch-or-build.sh to create PATH shim at ~/.local/bin/herdr-file-viewer.exe
• Removed deprecated Windows-only scripts: open-file-viewer.ps1, open-file-viewer-tab.ps1
• Updated herdr-plugin.toml with cross-platform pane and Windows-specific actions

Tests Updated

• tests/launcher_ps1.rs: Updated for new script names (open-file-viewer-windows.ps1, open-file-viewer-tab-windows.ps1)
• tests/launcher_content.rs: Updated script names, validated call-operator spawn, UTF-8 JSON handling, config dir passing
• All tests pass including cargo test launcher_ps1 and cargo test launcher_content

Backward Compatibility

• Version detection: checks herdr --version, uses native if >= v0.8.0
• Fallback for older herdr: pane split + pane run + send-keys

Testing

• All cargo test pass
• Manual verification: split pane, tab pane both work natively (pane PID 1 = herdr-file-viewer, no shell)
• Backward compatibility fallback tested manually