Include current stations in the XTide TCD build

The XTide TCD build ships tide stations only. packages/tcd/build.ts filters out every station whose kind is current before writing harmonics.txt and offsets.xml, so the 2,586 NOAA current stations in the unified database never reach XTide, OpenCPN, or any other libtcd reader. Raised on https://github.com/openwatersio/tide-database/pull/131#discussion_r3996988130 and deferred to keep that PR to the data model.

libtcd has carried current stations since the original harmonics files, so this is a mapping exercise rather than a format change. A reference current is a harmonic record whose level units are knots, with flood_begins and ebb_begins for the slack times and min_direction and max_direction for the ebb and flood headings in degrees true. A subordinate current is an offsets record against a reference current, with time adds for the flood and ebb extremes and level multipliers for the speed ratios. 881 of the current stations are reference records and 856 of those carry harmonics; the 25 without are identity-only and should be skipped the same way the identity-only CHS tide ports are.

Things to settle while doing it:

• Map flood_direction, ebb_direction, and mean_flow onto the libtcd direction and level fields, and confirm what XTide does with a reference current that has no mean flow.
• Map the subordinate offsets (slack_before_flood, slack_before_ebb, flood_time, ebb_time, flood_speed_ratio, ebb_speed_ratio) onto the offsets.xml fields, and decide what to write when a field is flagged missing.
• Keep current reference stations out of the tide master constituent list where their constituent sets differ, or confirm build_tide_db accepts one shared list.
• Extend the tcd tests to cover a current reference and a current subordinate in both unit systems, and check the metric build reports speeds in knots rather than converting them.
• Check the result loads in XTide and OpenCPN, since #106 shows readers are stricter than build_tide_db.

Start in packages/tcd/build.ts and trace how current stations are filtered before harmonics.txt and offsets.xml are written. Read the existing tide and CHS identity-only handling, then inspect the tcd tests and build_tide_db behavior for unit systems and constituent lists. Done means current reference and subordinate records map correctly, missing fields are handled, tests cover both unit systems, and the output loads in XTide and OpenCPN.