Drop str() casts in replace/replace_dict and fix callers that pass non-string replacements

`replace` / `replace_dict` are typed `Dict[str, str]`, but some callers pass other types. `symbolic.safe_replace` hands its callback sympy symbols (default `value_as_string=False`), and `LoopUnroll` passes the loop index as a number. #2583 converts these back with `str()` where names are stored (`LoopRegion.loop_variable`, interstate edge assignment keys). That also silently stores non-identifiers such as `"3"`.

Proposal: remove the `str()` casts, make `replace`/`replace_dict` reject non-string replacement values, and fix the offending callers at the source. This touches many call sites and needs a full CI run (see the discussion in #2583).

Locate replace and replace_dict, then trace the callers named in the issue: symbolic.safe_replace and LoopUnroll, along with the storage points discussed in #2583. Make replacement values string-only at the replacement functions, correct the offending callers at their source, and run the full CI suite to verify that non-identifiers are no longer silently stored.

Written by the indexing model from the issue text.