Mapping: each and when blocks

[alvinreal]

Description

Implement each (iterate over array elements) and when (conditional blocks).

TDD Tests

each

• each .items { rename .x -> .y } applies to every element
• each .items { set .total = .price * .qty } computed fields per element
• each on non-array → error
• each with multiple operations inside block
• Nested each: each .groups { each .members { ... } }
• each on empty array → no-op

when

• when .type == "admin" { set .perms = [...] } applies conditionally
• when condition false → block skipped
• when with else (if we support it, or defer)
• Multiple when blocks → all evaluated independently
• Nested when inside each
• when inside each: per-element conditional

Part of #3