refactor(parser): redistribute parser tests by layer

41 tests in parser/mod.rs moved next to the implementation:
  lexer.rs:                 7  (test_lexer_*: numbers, ops, identifiers,
                                strings, ampersand, error handling)
  parser_impl.rs:          18  (test_parser_*: precedence, associativity,
                                function calls, nested calls, mixed ops)
  evaluator.rs:            13  (test_expression_evaluator_*, function_registry,
                                complex expressions, error handling)
  registry_fns/viz.rs:      3  (test_sparkline_*)
  mod.rs (kept core):       0  (all parser-mod tests redistributed; the
                                file's remaining tests were already moved
                                to submodules from prior phases)

parser/mod.rs dropped from 1630 → 615 lines (now just the AST types,
Value/ErrorKind enums, lexer/registry/parser_impl/evaluator
declarations, and shared helpers).

All tests still pass: 491 unit + 136 PTY = 627 total.

Author: SamuelSchlesinger