Move the MIKE+ station lookup out of #702

#702 carries a MIKE+ database lookup that places measured timeseries in the network:
`_MikePlusStationResolver` (~290 lines in `obs.py`), `_observations_from_mikeplus`, the `db=`
and `source=` arguments on `NodeObservation.from_multiple` and
`ReachObservation.from_multiple`, `tests/test_mikeplus.py` (558 lines), and a section of the
user guide. None of it is needed for the mikeio1d move #702 is about (ADR-013, Phase 2), and
it is not ready to ship on its own terms.

What needs more thought:

- **No fixture.** Every test builds its own sqlite database with `build_db`, so the schema
assumptions are checked only against our own construction of them. Nothing here has been
read against a database MIKE+ wrote.
- **The `locationtype` codes are magic numbers.** `_NODE_TYPES = {8, 12}`,
`_LINK_TYPES = {9}`. Any other code is dropped and named only on the failure path. No
source says these are the codes, or that they are stable across MIKE+ versions.
- **`resitemname` is split on `;` and the first field taken.** An undocumented encoding.
- **`tsfilename` holds a Windows path and is matched on basename alone, case-folded.** Two
result files with the same name in different folders collide; `source=` is the only escape.
- **The nine columns across `m_Station` and `m_Measurement` are validated against one
layout.** The error says "The database layout is not the one modelskill expects" without
recording which layout that is.
- **`assetname` becomes the observation name only when it is unique across the selection.**
The names a user gets depend on what else is in the database.

`from_multiple(db=...)` is public API, so whatever shape lands ships with 1.4.0.

Extract it onto its own branch, off #702 or off `main` once #702 lands.

Start by reading obs.py, tests/test_mikeplus.py, the relevant user-guide section, and ADR-013 alongside issue #702. Identify the MIKE+ lookup code and API surface that must be separated, then review the fixture, schema, path, naming, and locationtype concerns before choosing the branch point. Done means the lookup is isolated from the #702 move with its public API and tests documented for the remaining design decisions.

Written by the indexing model from the issue text.