Uniform builder slicing with docstrings and default preprocessor pop

[Michael628]

Parent PRD

#15

What to build

Refactor build_composite_config in pyfm/core/builder.py so all three container types (SIMPLE, LIST, DICT) follow a uniform _preprocessor slicing pattern, clean up dead code, update the default preprocessor, and add docstrings.

Specific changes:

Builder (pyfm/core/builder.py):

• DICT branch: Add _preprocessor routing — each child receives processed_params | {"_preprocessor": _preprocessor[subconfig_label][key]} for its per-key slice
• LIST branch: Remove processed_params[subconfig_label] merging. Iteration structure comes entirely from _preprocessor[subconfig_label] (a list). Remove implicit single-item fallback and dict-to-list coercion (user story 9)
• Remove removesuffix("_config") dead code in LIST branch
• Add docstrings to build_composite_config and inner builder functions describing the _preprocessor contract per container type (user story 7)

Default preprocessor (pyfm/tasks/register.py):

• Change params.get("_preprocessor", {}) → params.pop("_preprocessor", {}) so preprocessor data is consumed (user story 5)

Tests (test/core/test_builder.py):

• Add a DICT container test fixture (currently missing)
• DICT: builder iterates _preprocessor[field_name] as a dict, builds one child per key
• DICT: empty dict produces empty child dict
• DICT: each child receives its own per-key preprocessor slice
• LIST: empty list produces empty child list
• LIST: verify no implicit coercion (list input required)
• Verify removesuffix removal doesn't break anything

Acceptance criteria

• All three container types use _preprocessor[subconfig_label] uniformly
• removesuffix("_config") removed from builder
• LIST branch no longer merges processed_params[subconfig_label] or coerces non-list input
• DICT branch routes per-key _preprocessor slices to children
• Default preprocessor uses pop instead of get
• Builder functions have docstrings describing _preprocessor contract
• DICT test fixture added and all new tests pass
• Existing tests still pass

Blocked by

None - can start immediately

User stories addressed

• User story 1 (clear preprocessor contract)
• User story 2 (uniform slicing pattern)
• User story 3 (single source of truth from _preprocessor)
• User story 4 (key by exact field name, no removesuffix)
• User story 5 (default preprocessor pop)
• User story 7 (docstrings)
• User story 9 (no implicit LIST coercion)
• User story 10 (DICT per-key routing)