[BUG] Fix titan numbers in forest wave gamemode

Forest wave gamemode titan counts differ from AoTTG1. The wave settings should match the
expected progression: start with 3 titans and increase by 1 each round, except on boss
waves 5, 10, 15, and 20.

Requested behavior:
- Forest wave starts at 3 titans.
- Normal waves increase by 1 titan per wave.
- The normal-wave count should use the current wave offset correctly, so wave 2 spawns
  Start + (Wave - 1) * Increment, not Start + Wave * Increment.
- Boss waves should spawn boss titans through the boss spawning path rather than using
  the normal titan count calculation.

--- Original GitHub issue (auto-fetched) ---
Title: [BUG] Fix titan numbers in forest wave gamemode

Body:
**How will this enhancement improve the game?**
The forest wave gamemode settings are different to AoTTG1, and should ideally be the same.

**Describe the solution you'd like**
- [x] Change the Titans per wave in Forest back to the correct amount (starts at 3, increases by 1 each round except for waves 5, 10, 15, and 20). If this is something controlled by the new spawn service that’s not yet been finished then it can wait (Forest already doesn’t go past wave 5 due to the lack of this new spawn service).

**Describe alternatives you've considered**
@Acros5542 can provide more feedback on the issue if needed.

**Contributor instructions** (Do not modify)
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2. If the idea is denied, please provide a reason and close the issue. If the idea is accepted, consult your team to do an estimation of effort. Remove the [PROPOSAL] name and label.
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