feat: add -m, -e, and --dry-run CLI mapping flags

[alvinreal]

Summary

Adds mapping support to the CLI with three new flags, completing issue #19.

New Flags

• -m / --mapping <file> — Load and apply a .morph mapping file
• -e / --expr <expression> — Inline mapping expression (repeatable, applied in order)
• --dry-run — Parse and validate the mapping without executing

Behavior

• Multiple -e flags combine in order
• -m and -e can be combined: file operations apply first, then inline expressions
• No mapping flags → straight passthrough conversion (unchanged behavior)

Tests

Unit tests (in src/cli.rs):

• arg_parsing_mapping_file — -m flag parses correctly
• arg_parsing_single_expr — Single -e flag
• arg_parsing_multiple_expr — Multiple -e flags
• arg_parsing_dry_run — --dry-run flag
• arg_parsing_mapping_and_expr_combined — -m + -e together
• no_mapping_flags_returns_none — No flags → None
• build_mapping_from_expr — Builds program from -e
• build_mapping_multiple_exprs_in_order — Multiple exprs produce correct statement count
• build_mapping_invalid_expr_returns_error — Bad syntax → error
• build_mapping_nonexistent_file_returns_error — Missing file → error

Integration tests (in tests/cli_mapping.rs):

• mapping_file_applies_transformations — -m with valid file
• mapping_file_nonexistent_gives_error — -m with missing file
• mapping_file_invalid_syntax_gives_parser_error — -m with bad syntax
• single_expr_works — -e with one expression
• multiple_expr_apply_in_order — Multiple -e in sequence
• mapping_and_expr_combined — -m + -e end-to-end
• dry_run_valid_mapping_reports_valid — --dry-run with valid mapping
• dry_run_invalid_mapping_reports_error — --dry-run with bad mapping
• no_mapping_flags_passthrough — Passthrough without mapping
• full_integration_json_to_yaml_with_mapping — Full end-to-end with file I/O

Fixes #19