Warmup to cosine scheduler handoff can overshoot peak LR

## Problem

At the warmup/cosine transition, the scheduler wrapper can delegate to `CosineAnnealingLR.get_lr()` while the cosine scheduler is in a chainable state with `last_epoch == 0` and the optimizer LR already set to the warmup peak.

That state applies the recursive cosine update to the current peak LR instead of returning the epoch-0 peak value.

## Impact

The learning rate can exceed the configured peak for one step. With `eta_min = 0`, the transition factor is:

```text
2 / (1 + cos(pi / T_max))
```

For example, `T_max = 10` overshoots by about 2.51%.

## Fix

Fixed in PR #30 by special-casing the handoff to return the configured peak/base LRs directly, then letting the following scheduler step advance cosine normally.