ARM: a three-cell horizontal trace
Trace ARM from row 1, column 5 and move left twice. The full sequence is 1 at row 1, column 5; 2 at row 1, column 4; 3 at row 1, column 3. Reading those cells gives A → R → M, with no detour or repeated cell. The A at row 1, column 5 begins the entry, and the M at row 1, column 3 closes it. If you suspected the three-letter word, this compact run is the quickest exact confirmation.
This route uses two left moves and no turn, so the visual check is simple: all three letters sit together on the top edge. More importantly, it does not consume the H and T at the other end of that row. Those letters can still connect downward through the open left side, while 16 cells remain for the 4-, 5-, and 7-letter routes. If an ARM candidate strands either top-left letter, reject that candidate before trying to repair the rest of the board.