docs: clarify v2.1.0 conformance requirements

[ChelseaKR]

Summary

• remove alignment padding from CSV examples so copied values do not contain unintended spaces
• use the defined start_mid_trip and end_mid_trip field names throughout the run-events prose
• spell out all four employee_run_dates.txt primary-key fields
• state which GTFS Required fields an added Supplement row must provide
• scope the conditional vehicle_assignments.service_id requirement to each row's block_id

Issue #152 disposition

This implements each clarification confirmed in #152:

1. Example whitespace: removes the padding from every affected CSV example. Spaces that are part of a field value, such as Report Time, are unchanged.
2. Mid-trip field names: replaces mid_trip_start and mid_trip_end in prose with the existing table fields, start_mid_trip and end_mid_trip.
3. Employee assignment primary key: replaces Primary Key: * with (date, service_id, run_id, employee_id), matching GTFS's meaning of * while making the constraint explicit.
4. Added Supplement rows: states that a row being added must define all fields marked Required in the corresponding GTFS file, plus any Conditionally Required field whose condition applies to that row. Updates and deletes still need only the Primary Key fields.
5. Vehicle assignment service: makes service_id conditionally required only for a row whose block_id occurs under more than one service_id.

Scope

This PR does not change the Time definition. That clarification was handled in #150 and is already on main.

The changes clarify the intended behavior discussed in #152; they do not add new files or fields.

Validation

• make build
• git diff --check

Closes #152.

[skyqrose]

LGTM and satisfies all the discussion from #152