Add validation for invalid costume and backdrop names

[aspizu]

Summary

• Adds compile-time validation for switch_costume and switch_backdrop blocks
• Warns when costume/backdrop names don't exist, preventing runtime errors
• Provides helpful guidance when file extensions are incorrectly included

Changes

• Added InvalidCostumeName and InvalidBackdropName diagnostic types
• Enhanced switch_costume validation against sprite's costume list
• Enhanced switch_backdrop validation against stage's costume list (backdrops)
• Cross-sprite backdrop validation: properly accesses stage from any sprite context
• Helpful error messages with file extension guidance

Test plan

• Validates invalid costume names in switch_costume blocks
• Validates invalid backdrop names in switch_backdrop blocks
• Shows helpful error messages with file extension guidance
• Works from both stage and non-stage sprites
• Existing tests pass with new validation errors caught

Example Output

error: costume 'dango' does not exist
--> main.gs:8:20
|
8 |     switch_costume "dango";
  |                    ^^^^^^^

error: backdrop 'dango' does not exist  
--> main.gs:11:21
|
11 |     switch_backdrop "dango";
   |                     ^^^^^^^

Closes #156

🤖 Generated with Claude Code

[faretek1]

does this incorrectly warn against 'previous costume' etc?

[aspizu]

you should use the previous/next blocks instead