fix: keep calcdirsize results when directory is reloaded

[tk1475]

Fixes #2675.

set dircounts false calls nav.renew(), which starts an async reload of the directory because the dircounts value changed. calcdirsize then runs right away and stores the sizes on the current file objects. When the reloaded directory arrives on dirChan, it replaces the cached one with fresh file objects whose dirSize is -1, so the sizes flash on screen and then turn back into -.

This adds a small copyDirSizes helper on dir that carries over already calculated sizes by path for directories that are still present, and calls it in the dirChan handler when an existing directory is replaced (before sorting, so sortby size keeps using them).

Testing

• New TestCopyDirSizes in nav_test.go. With a no-op helper it fails (at "a" expected dirSize 42 but got -1), and passes with the fix.
• Reproduced end to end by driving the built binary in a pseudo terminal with map C :set dircounts false; calcdirsize and pressing C once: on master the directory shows -, with this change it shows 3.0M.
• go fmt, go vet ./..., GOOS=windows go vet ./..., go test ./..., golangci-lint run ./... (0 issues).

This change was investigated and written with the help of an AI assistant (Claude Code); the checks above were run locally.

🤖 Generated with Claude Code

[valoq]

The fix works for #2675 but sizes are copied by path only. If a directory is deleted and created again or a file is added to it, it keeps its old size after a reload. you could copy the size only if the directory's modification time did not change.

[tk1475]

@valoq thanks, good point. Pushed d8078b0: a cached size is now only copied over if the directory's modification time matches the previous load, so a recreated directory or one with entries added or removed gets recalculated. The test covers the changed mtime case too.

One limit: a directory's mtime only changes when its direct entries change, so a file added deeper in the tree (say a/b/new) keeps the old size for a until calcdirsize is run again. Checking that would mean walking the tree on every reload, which is what calcdirsize is there to avoid. Happy to change it if you'd prefer a different trade-off.