feat: add comprehensive icon validation to mcpb validate command

Implements icon validation to catch common issues that prevent icons from
displaying in Claude Desktop. Uses a non-blocking warning approach for
remote URLs to maintain flexibility for MCPB's evolving multi-platform
ecosystem.

## Changes

### Core Implementation (src/node/validate.ts)
- Add `isPNG()` helper to validate PNG file signatures via magic bytes
- Add `validateIcon()` function with comprehensive checks:
  - **Warn** about remote URLs (non-blocking, best practice guidance)
  - **Reject** ${__dirname} variable usage in icon field (doesn't work)
  - **Reject** absolute paths (not portable)
  - Verify icon file exists at specified path
  - Verify icon file is valid PNG format
  - Recommend 512×512 size for Claude Desktop
- Integrate icon validation into `validateManifest()` after schema validation

### Test Coverage
- Update `test/icon-validation.test.ts` - Remote URL test now expects warning
- Update `test/cli.test.ts` - Integration test updated for warnings
- All 109 tests passing

### Documentation Updates
- Update examples/hello-world-node/manifest.json to v0.3

## Validation Behavior

**Remote URLs (WARNING - non-blocking):**
```
Icon path uses a remote URL. Best practice for local MCP servers:
Use local files like "icon": "icon.png" for maximum compatibility.
Claude Desktop currently only supports local icon files in bundles.
```
**Result:** Validation passes, developer is informed

**Other Issues (ERROR - blocking):**
- ${__dirname} usage: Rejects (doesn't work)
- Absolute paths: Rejects (not portable)
- Missing files: Rejects (broken reference)
- Non-PNG files: Rejects (wrong format)

## Rationale

**Non-Blocking Approach:**
- Doesn't prevent valid bundles from being created
- Educates developers about best practices
- Maintains flexibility as MCPB evolves beyond Claude Desktop
- Future-proof if remote URL support is added

**Still Protective:**
- Blocks truly broken configurations (missing files, wrong formats)
- Provides actionable error messages for fixable issues

Based on real-world findings from investigating icon display issues in
Claude Desktop with production MCPB bundles.

schemas/mcpb-manifest-v0.2.schema.json

@@ -6,12 +6,12 @@
     },
     "dxt_version": {
       "type": "string",
-      "const": "0.2",
+      "const": "0.3",
       "description": "@deprecated Use manifest_version instead"
     },
     "manifest_version": {
       "type": "string",
-      "const": "0.2"
+      "const": "0.3"
     },
     "name": {
       "type": "string"
@@ -80,12 +80,54 @@
     "icon": {
       "type": "string"
     },
+    "icons": {
+      "type": "array",
+      "items": {
+        "type": "object",
+        "properties": {
+          "src": {
+            "type": "string"
+          },
+          "sizes": {
+            "type": "string",
+            "pattern": "^\\d+x\\d+$"
+          },
+          "theme": {
+            "type": "string",
+            "minLength": 1
+          }
+        },
+        "required": [
+          "src",
+          "sizes"
+        ],
+        "additionalProperties": false
+      }
+    },
     "screenshots": {
       "type": "array",
       "items": {
         "type": "string"
       }
     },
+    "localization": {
+      "type": "object",
+      "properties": {
+        "resources": {
+          "type": "string",
+          "pattern": "\\$\\{locale\\}"
+        },
+        "default_locale": {
+          "type": "string",
+          "pattern": "^[A-Za-z0-9]{2,8}(?:-[A-Za-z0-9]{1,8})*$"
+        }
+      },
+      "required": [
+        "resources",
+        "default_locale"
+      ],
+      "additionalProperties": false
+    },
     "server": {
       "type": "object",
       "properties": {
@@ -313,6 +355,13 @@
         ],
         "additionalProperties": false
       }
+    },
+    "_meta": {
+      "type": "object",
+      "additionalProperties": {
+        "type": "object",
+        "additionalProperties": {}
+      }
     }
   },
   "required": [