Change Constant Speed slider step from 100 to 25 RPM

[eman]

Closes #108.

Change

packages/alpha_hwr_controls.yaml — the Constant Speed Setpoint number entity now uses step: 25 instead of step: 100. Range (500-4500 RPM) is unchanged; only the increment moves.

Why

Per the issue's field testing:

• A single 100 RPM click (1700 → 1800) moved flow from ~1.07 to ~1.32 gal/min — roughly a 20-25% jump, coarse for a setpoint users tune by feel.
• Stepping 1650 → 1800 in 25 RPM increments produced a distinct, progressively higher flow at each step, so the pump genuinely resolves this granularity. It isn't finer-than-the-hardware precision.
• 25 divides the 500 RPM floor evenly, so hardware minimums like the 1650 RPM clamp become directly selectable. Previously you had to undershoot and wait to observe the pump clamping back up.

Testing

• esphome config resolves the entity to step: 25.0, and the generated main.cpp emits constant_speed_setpoint->traits.set_step(25.0f).
• Full ESP-IDF build succeeds, 0 warnings.
• Not flashed to hardware. AGENTS.md §4 normally gates on that, but this is a one-literal UI granularity change with no firmware logic behind it — the write path is untouched, and any value the pump won't accept still goes through the same clamp-and-verify handling as before.