grid.ticks increments by 2 per tick() (pre-existing)

[gilesknap]

Summary

World::grid.ticks advances by 2 for every logical tick, not 1.

This is long-standing behaviour, not introduced by the multithreading work. The original World::tick() incremented grid.ticks at both its start and end:

pub fn tick(&mut self) {
    self.grid.ticks += 1;   // start
    // ... think + grass ...
    self.apply_updates();
    self.grid.ticks += 1;   // end
}

The think/resolve refactor (PR #5) preserved this exactly: one increment now lives in think_phase() and one in resolve_phase(), so tick() still yields +2.

Why it matters

grid.ticks is used by:

• Grass-growth scheduling — next_grass_tick is compared against grid.ticks, so grass effectively grows on a tick scale that is 2× the logical tick count (and ticks_per_grass() is implicitly calibrated to the +2 cadence).
• The GUI tick readout and ticks/sec display.
• Saved-world tick counts (serialized in WorldGrid).

Decision needed

Changing it to +1 is a one-line removal of the resolve_phase() increment, but it shifts grass cadence and the meaning of ticks, and would want the grass timing / benchmarks re-checked. Filing this so the change can be made deliberately rather than bundled into the threading PR.

Flagged by CodeRabbit on PR #5.