fix: correct error message punctuation per style guide

- Add periods to Error messages for argument validation
- Maintain no-period style for PurlError messages
- Follows CLAUDE.md error handling patterns

Author: jdalton

src/objects.ts

@@ -29,7 +29,7 @@ function recursiveFreeze<T>(value_: T): T {
   while (pos < queueLength) {
     // Safety check to prevent processing excessively large object graphs.
     if (pos === LOOP_SENTINEL) {
-      throw new Error('Object graph too large (exceeds 1,000,000 items)')
+      throw new Error('Object graph too large (exceeds 1,000,000 items).')
     }
     const obj = queue[pos++]!
     Object.freeze(obj)

src/package-url.ts

@@ -279,7 +279,7 @@ class PackageURL {
 
     // Validate parsed result is an object.
     if (!parsed || typeof parsed !== 'object' || Array.isArray(parsed)) {
-      throw new Error('JSON must parse to an object')
+      throw new Error('JSON must parse to an object.')
     }
 
     // Cast to record type for safe property access
@@ -304,7 +304,7 @@ class PackageURL {
    */
   static fromObject(obj: unknown): PackageURL {
     if (!isObject(obj)) {
-      throw new Error('Object argument is required')
+      throw new Error('Object argument is required.')
     }
     const typedObj = obj as Record<string, unknown>
     return new PackageURL(
@@ -333,7 +333,7 @@ class PackageURL {
   static parseString(purlStr: unknown): unknown[] {
     // https://github.com/package-url/purl-spec/blob/master/PURL-SPECIFICATION.rst#how-to-parse-a-purl-string-in-its-components
     if (typeof purlStr !== 'string') {
-      throw new Error('A purl string argument is required')
+      throw new Error('A purl string argument is required.')
     }
     if (isBlank(purlStr)) {
       return [undefined, undefined, undefined, undefined, undefined, undefined]