refactor[next]: Replace apply_to_primitive_constituents with tree_map_type

[havogt]

Closes #1570

Description

Removes type_info.apply_to_primitive_constituents and migrates all call sites to tree_map_type, completing the long-standing TODO in ffront/type_info.py.

• The canonical tree_map_type / _tree_map_type_constructor are moved from ffront.type_info down to the low-level type_system.type_info, so iterator- and codegen-layer call sites can use them without an upward dependency on ffront; ffront.type_info re-exports them.
• Unlike apply_to_primitive_constituents (which recursed only on ts.TupleType and treated ts.NamedCollectionType as a leaf), tree_map_type recurses over COLLECTION_TYPE_SPECS and preserves named-collection structure. This changes behavior only for NamedCollectionType inputs, which are lowered to plain tuples (named_collections_to_tuple_types) before any iterator-/codegen-stage call site — so the migrated sites are unaffected.
• tree_map_type is overloaded so its result type reflects the constructor's return (_T | ts.CollectionTypeSpec for the default, _T | _C for a custom result_collection_constructor) instead of tree_map's intentionally weak return type, avoiding casts at the call sites.

Pure refactoring, no behavior change at any call site.

Requirements

• All fixes and/or new features come with corresponding tests. — behavior-preserving refactor, covered by existing suites (type system, iterator type inference, global_tmps, gtfn codegen, ffront type deduction).
• Important design decisions have been documented in the appropriate ADR — N/A, no user-facing design change.

[Copilot]

Pull request overview

This PR completes the refactor away from type_info.apply_to_primitive_constituents by removing it and migrating all call sites to the more general tree_map_type, while relocating the canonical tree_map_type implementation to the lower-level next.type_system.type_info module (and re-exporting it from ffront.type_info) to avoid upward dependencies.

Changes:

• Removed apply_to_primitive_constituents and introduced/standardized tree_map_type in next.type_system.type_info as the canonical API.
• Migrated iterator, codegen, and frontend type-deduction call sites to tree_map_type, including updating custom collection constructors to match tree_map’s (value, elts) constructor convention.
• Tightened/clarified typing at a few call sites (e.g., explicit assertions for tuple/field results, adjusted return type annotation in type_synthesizer).

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/gt4py/next/type_system/type_info.py	Removes apply_to_primitive_constituents; adds canonical _tree_map_type_constructor + tree_map_type wrapper around next.utils.tree_map.
src/gt4py/next/program_processors/codegens/gtfn/itir_to_gtfn_ir.py	Switches tuple-leaf processing to tree_map_type and updates tuple constructors to the (value, elts) convention.
src/gt4py/next/iterator/type_system/type_synthesizer.py	Replaces dtype/field construction mappings with tree_map_type; updates return typing and adds sanity assertions.
src/gt4py/next/iterator/type_system/inference.py	Updates temporary type construction to use tree_map_type and asserts expected mapped result shape.
src/gt4py/next/iterator/transforms/global_tmps.py	Migrates tuple-structured name/dtype/domain propagation logic to tree_map_type with an explicit tuple constructor.
src/gt4py/next/iterator/transforms/fuse_as_fieldop.py	Replaces dtype extraction mapping with tree_map_type for list-type checks.
src/gt4py/next/ffront/type_info.py	Re-exports canonical _tree_map_type_constructor and tree_map_type from next.type_system.type_info.
src/gt4py/next/ffront/foast_passes/type_deduction.py	Migrates astype return type mapping to tree_map_type and removes an unnecessary type-ignore on where return-type deduction.