feat: parallel and map branch name

[wangyb-A]

Issue #, if available:
close #303

Description of changes:

Add support for custom naming of map iterations and parallel branches.

Map:

• item_namer on MapConfig[T]
• Make MapConfig generic over T (the item type)
• Add optional item_namer: Callable[[T, int], str] field
• When provided, replaces the default map-item-{index} naming scheme
• Receives the item and its index, returns a custom name string

Parallel:

• Add ParallelBranch frozen dataclass with func and optional name
• ParallelBranch implements call, making it directly usable anywhere a plain callable is accepted
• When name is provided, replaces the default parallel-branch-{index} naming in execution history

Shared mechanism:

• Add get_iteration_name(index) method to ConcurrentExecutor base class
• MapExecutor and ParallelExecutor override it for custom naming

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Testing: