refactor: introduce ensure_value_choice() helper for enum-like string parameter validation

[murray-ds]

Problem

The codebase has 14+ locations that independently validate string parameters against a fixed set of allowed values (sort_order, agg_func, orientation, data_label_format, etc.). This leads to:

• Active bugs — composite_rank.py validates sort_order and agg_func with .lower() but uses the raw value in branching/lookup logic. sort_order="ASC" passes validation but silently produces descending ranks. agg_func="MEAN" passes validation but raises a KeyError on dict lookup.
• Inconsistent error messages across modules
• 3-5 lines of boilerplate duplicated at each call site

Bug example (composite_rank.py:260-264)

if sort_order.lower() not in valid_sort_orders:  # "ASC" passes validation
    ...
order_by = ibis.asc(df[col_name]) if sort_order in ["asc", "ascending"] else ibis.desc(df[col_name])
# "ASC" falls through to else → silently returns descending order

Proposal

Add ensure_value_choice() to pyretailscience/core/validation.py (alongside ensure_columns() from #428). A single function that validates a string value against allowed choices, optionally normalizing case, and returns the validated value.

Call Sites

Bugs (validate with .lower(), use raw value)

File	Parameter	Lines
analysis/composite_rank.py	sort_order	260-264
analysis/composite_rank.py	agg_func	351-355

Normalize-first (correct but duplicated)

File	Parameter	Lines
plots/waterfall.py	data_label_format	86-94
plots/index.py	agg_func	392-394

Exact match (correct but boilerplate)

File	Parameter	Lines
plots/bar.py	orientation, sort_order, data_label_format	102-117
plots/index.py	sort_by, sort_order	248-253
plots/histogram.py	range_method	183-185
segmentation/segstats.py	orientation, sort_order	616-619
analysis/cohort.py	period	73-75

Acceptance Criteria

• ensure_value_choice() exists in pyretailscience/core/validation.py
• Full test coverage for the helper
• Both composite_rank.py bugs are fixed
• All call sites above are migrated
• All existing tests pass
• Error messages are consistent across the codebase after migration