Add "non-material differences" to the schema

[jcpitre]

We should have a way to signal differences between GTFS datasets that do not affect the outcome but that may still interest a user. These are non-material differences — the data is
semantically equivalent, but the raw text differs.

Examples:

• Column position changed
• Row position changed
• Numeric formatting differences — e.g. 45.5000 vs 45.5
• Quoting differences — e.g. S1,Main St,Downtown vs S1,"Main St","Downtown" (both are valid CSV)
• Time leading zeros — e.g. 8:30:00 vs 08:30:00
• etc.

A key use case: if gtfs-diff deems two files semantically identical (e.g. stop_times.txt) but a raw string diff shows differences, we should be able to explain the discrepancy by surfacing which non-material differences were detected. This helps users understand why the diff engine reports no changes even though the files are not byte-for-byte identical.