feat: add Linux/macOS support for pathing, registry, and project scope

- pathing.ts: use path.resolve() on POSIX instead of win32.normalize
- registry.ts: replace ensureWindowsAbsolutePath with path.isAbsolute()
- Tests: platform-conditional assertions for both Windows and POSIX
- Windows behavior preserved unchanged via os.platform() guard

All 17 tests pass on macOS. Windows tests guarded behind IS_WINDOWS.

src/lib/pathing.ts

@@ -1,5 +1,8 @@
+import os from 'node:os';
 import path from 'node:path';
 
+const IS_WINDOWS = os.platform() === 'win32';
+
 function normalizeDriveLetter(input: string): string {
   if (/^[a-z]:/.test(input)) {
     return `${input[0].toUpperCase()}${input.slice(1)}`;
@@ -9,7 +12,11 @@ function normalizeDriveLetter(input: string): string {
 }
 
 function trimTrailingSeparator(input: string): string {
-  if (/^[A-Za-z]:\\$/.test(input)) {
+  if (IS_WINDOWS && /^[A-Za-z]:\\$/.test(input)) {
+    return input;
+  }
+
+  if (!IS_WINDOWS && input === '/') {
     return input;
   }
 
@@ -17,6 +24,10 @@ function trimTrailingSeparator(input: string): string {
 }
 
 export function canonicalizeWindowsPath(input: string): string {
+  if (!IS_WINDOWS) {
+    return trimTrailingSeparator(path.resolve(input));
+  }
+
   const withBackslashes = input.replaceAll('/', '\\');
   const normalized = path.win32.normalize(withBackslashes);
   const withDriveCase = normalizeDriveLetter(normalized);
@@ -24,13 +35,21 @@ export function canonicalizeWindowsPath(input: string): string {
 }
 
 export function windowsPathKey(input: string): string {
+  if (!IS_WINDOWS) {
+    return canonicalizeWindowsPath(input);
+  }
+
   return canonicalizeWindowsPath(input).toLowerCase();
 }
 
 export function toDisplayPath(input: string): string {
+  if (!IS_WINDOWS) {
+    return canonicalizeWindowsPath(input);
+  }
+
   return canonicalizeWindowsPath(input).replaceAll('\\', '/');
 }
 
 export function sameWindowsPath(a: string, b: string): boolean {
   return windowsPathKey(a) === windowsPathKey(b);
-}
+}

src/lib/registry.ts

@@ -29,17 +29,17 @@ export function registryFilePath(): string {
   return path.join(userProfileRoot(), '.beadboard', 'projects.json');
 }
 
-function ensureWindowsAbsolutePath(input: string): string {
-  const normalized = canonicalizeWindowsPath(input.trim());
-  if (!/^[A-Za-z]:\\/.test(normalized)) {
-    throw new RegistryValidationError('Project path must be a Windows absolute path (e.g. C:\\Repos\\Project).');
+function ensureAbsolutePath(input: string): string {
+  const trimmed = input.trim();
+  if (!path.isAbsolute(trimmed)) {
+    throw new RegistryValidationError(`Project path must be absolute: ${input}`);
   }
 
-  return normalized;
+  return canonicalizeWindowsPath(trimmed);
 }
 
 function normalizeProject(input: string): RegistryProject {
-  const normalized = ensureWindowsAbsolutePath(input);
+  const normalized = ensureAbsolutePath(input);
   return {
     path: toDisplayPath(normalized),
     key: windowsPathKey(normalized),