Epic M: consume file pointers from external metadata sources (skip filefinder)

[jepegit]

Context

Epic M read path is in place: MetadataSource Protocol + MetaResolver hook (#784, merged via #1106) and the BatBase adapter (cellpy/cellpy-connectors#2). BatBase is getting pointers to the actual data files on each experiment (ife-bat/batbase#474: files: [{kind, uri, order, size, mtime, checksum, …}] on the journal API).

Today cellpy still finds raw files and .cellpy archives with filefinder (glob over rawdatadir / cellpydatadir by cell name). When the metadata source already knows where the files are, that step is wasted — slow on network shares and brittle across machines.

Spec

1. Contract: add MetaRecord.files: tuple[FileRef, ...] (FileRef(kind, uri, order=0, size=None, mtime=None, checksum=None, loader=None)). validate_record accepts it; raw_file_names / source_uri stay provenance (forbidden for sources). ExternalLink records that files were supplied.
2. Cell path: cellpy.get(source="batbase", key=…) (or CellpyCell.from_source) — fetch the record, open files[kind=="cellpy"] if present and fresh, else files[kind=="raw"] (ordered) with the loader hint, else fall back to today's filefinder. Uses OtherPath for remote URIs. Apply the metadata as fetch_meta does.
3. Batch path: batch.from_source("batbase", tag=…, project=…) builds the journal pages from the records (filename(s) from files, mass / area / nom_cap / label / cell_type / cycle_mode from the record); missing files ⇒ filefinder per cell as now. Provenance names the source per field (Resolution.origin_of).
4. Change detection: when size/mtime are present, update() / batch.refresh() may use them to skip stat-ing the raw file.
5. Adapter side (cellpy-connectors): map BatBase files → FileRefs (follow-up issue there once docs: Stage 0 foundations complete (#439) #474 ships).

Everything must keep working when a source returns no files.

Acceptance

• A record with files loads a cell without touching filefinder (asserted with a monkeypatched finder).
• A record without files behaves exactly as today.
• batch.from_source produces pages equivalent to the current journal for a tagged set, offline against DictMetadataSource.

Related

#784, #783 (Epic M), cellpy/cellpy-connectors#2, ife-bat/batbase#474, ife-bat/batbase#473. Pairs with M3 (push: cellpy registering the files it loaded back into BatBase).