Claude/windows install compatibility tr di s (#3)

* Fix Windows PowerShell 5.1 compatibility in installer

Use $env:PROCESSOR_ARCHITECTURE for arch detection instead of
RuntimeInformation::OSArchitecture which may not be loaded in
every Windows PowerShell 5.1 session. Also fix null-reference
when user PATH environment variable is empty.

https://claude.ai/code/session_01VFiRDM2ZweyacXN5JneVoP

* Fix executable resolution and tar extraction on Windows

resolveExecutable() used `sh -lc "command -v ..."` which doesn't work
on Windows (no sh). Now uses `cmd /c where` on win32. Also make tar
workspace restoration tolerate symlink failures on Windows — .bin/
symlinks can't be created without Developer Mode, but the actual
package directories are extracted fine.

https://claude.ai/code/session_01VFiRDM2ZweyacXN5JneVoP

* Broad Windows compatibility fixes across the codebase

- runtime.ts: Use path.delimiter instead of hardcoded ":" for PATH
  construction — was completely broken on Windows
- executables.ts: Add Windows fallback paths for Chrome, Edge, Brave,
  and Pandoc in Program Files; skip macOS-only paths on win32
- node-version.ts, check-node-version.mjs, bin/feynman.js: Show
  Windows-appropriate install instructions (irm | iex, nodejs.org)
  instead of nvm/curl on win32
- preview.ts: Support winget for pandoc auto-install on Windows, and
  apt on Linux (was macOS/brew only)
- launch.ts: Catch unsupported signal errors on Windows
- README.md: Add Windows PowerShell commands alongside macOS/Linux
  for all install instructions

https://claude.ai/code/session_01VFiRDM2ZweyacXN5JneVoP

* fix: complete windows bootstrap hardening

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Advait Paliwal <advaitspaliwal@gmail.com>

src/system/node-version.ts

@@ -26,10 +26,15 @@ export function isSupportedNodeVersion(version = process.versions.node): boolean
 }
 
 export function getUnsupportedNodeVersionLines(version = process.versions.node): string[] {
+	const isWindows = process.platform === "win32";
 	return [
 		`feynman requires Node.js ${MIN_NODE_VERSION} or later (detected ${version}).`,
-		"Switch to Node 20 with `nvm install 20 && nvm use 20`, or use the standalone installer:",
-		"curl -fsSL https://feynman.is/install | bash",
+		isWindows
+			? "Install a newer Node.js from https://nodejs.org, or use the standalone installer:"
+			: "Switch to Node 20 with `nvm install 20 && nvm use 20`, or use the standalone installer:",
+		isWindows
+			? "irm https://feynman.is/install.ps1 | iex"
+			: "curl -fsSL https://feynman.is/install | bash",
 	];
 }