from_res1d cannot reach EPANET link quantities — no breakpoint for single-gridpoint reaches

## Summary

`from_res1d` can't reach EPANET link quantities, because EPANET-style water distribution links never get a breakpoint to match against.

## Root cause

`Res1DReach.__init__` (`src/modelskill/model/adapters/_res1d.py:77-109`) only creates intermediate breakpoints when a reach has more than 2 gridpoints:

```python
intermediate_gridpoints = (
reach.gridpoints[1:-1] if len(reach.gridpoints) > 2 else [] # line 95-97
)
```

EPANET-style links typically have exactly 2 gridpoints (start and end only, no intermediate H-points), so `intermediate_gridpoints` is always empty and `Res1DReach.breakpoints` is always `[]` for these links. As a result, `NetworkModelResult._extract_reach` (`src/modelskill/model/network.py:212-281`) can never find data to match a `ReachObservation` against such a link, and always falls through to the `ValueError` at lines 277-281.

## Request

Treat a 2-gridpoint reach as having one implicit mid-reach breakpoint. This fits the existing `ReachBreakPoint`/`BasicReach` data model without changing it. A proof of concept for this already exists.

## Relevant extension points

- `ReachBreakPoint` (`src/modelskill/network.py:81-138`)
- `BasicReach` (`network.py:270-323`)

Found by a test user evaluating the network functionality ahead of a 1.4.0 release.

Start in src/modelskill/model/adapters/_res1d.py at Res1DReach.__init__, then review ReachBreakPoint and BasicReach in src/modelskill/network.py. Trace NetworkModelResult._extract_reach in src/modelskill/model/network.py; done means a two-gridpoint EPANET link exposes an implicit mid-reach breakpoint and its link quantities can be matched without the current ValueError.

Written by the indexing model from the issue text.