Epic M: use FileRef size/mtime from the metadata source to skip stat-ing raw files on update()

[jepegit]

Context

#1107 (PR #1119) added FileRef(size, mtime, checksum, …) on MetaRecord.files, and batch.from_source already carries them into the journal pages as raw_file_size / raw_file_mtime. Nothing consumes them yet: CellpyCell.update() / Batch.refresh() still stat every raw file (slow on scp:///SFTP shares) to decide whether anything changed.

Spec

1. When a cell was loaded via a source record whose raw FileRefs carry size and/or mtime, store them next to the back-link (ExternalLink or Data provenance — pick one, document it) so they survive save/load.
2. update() / refresh() / poll(): if the source's recorded size+mtime equal what cellpy already loaded, skip the remote stat and treat the file as unchanged. A changed value, or a missing one, falls back to today's stat-based path. checksum is informational only (no hashing of remote files).
3. Batch: Batch.refresh() may consult raw_file_size / raw_file_mtime the same way; batch.from_source(...).update() followed by a second from_source fetch can short-circuit per cell.
4. Never skip when the user forces a reload (recalc=True, RAW_ONLY, explicit update(force=…) if that exists).

Acceptance

• A cell loaded from a DictMetadataSource record with size/mtime does not call the path stat on update() when the values match (monkeypatched OtherPath.stat asserts not called), and does when they differ.
• Records without size/mtime behave exactly as today.
• Round-trip: the recorded values survive .cellpy save/load.

Related

#1107, #783 (Epic M), ife-bat/batbase#474 (size/mtime on TestDataFile), incremental-load protocol (#779 / #164).