feat: implement each and when block statements

[alvinreal]

Summary

Implements each and when block statements as specified in #23.

New Operations

each .path { ... } — Iterate over array elements, apply block to each

each .items { rename .x -> .y }
each .items { set .total = .price * .qty }
each .groups { each .members { rename .old -> .new } }

when <condition> { ... } — Conditional block execution

when .type == "admin" { set .perms = "all" }
each .users { when .role == "admin" { set .elevated = true } }

Behavior

• each: applies all statements in the block to every array element
• each on non-array: returns error
• each on empty array: no-op
• Nested each: fully supported for deeply nested structures
• when true: block is applied to the value
• when false: block is skipped, value unchanged
• Multiple when: all evaluated independently in sequence
• when inside each: per-element conditional logic

Tests (11 integration tests)

• each_rename_in_elements — Rename in every element
• each_computed_fields — Computed fields per element
• each_on_non_array_error — Error on non-array
• each_multiple_operations — Multiple ops in block
• nested_each — Nested each blocks
• each_empty_array — Empty array no-op
• when_condition_true — Block applied
• when_condition_false — Block skipped
• multiple_when_blocks — Independent evaluation
• when_inside_each — Conditional inside iteration
• each_with_when_conditional_per_element — Per-element conditional

Fixes #23