check always warns about files the manifest does not list (closes #103)

check now always looks under the base directory for files the manifest
does not list, hidden files and directories included, and prints one
warning per file. The result still depends only on the listed files;
--no-extra-files turns each unlisted file into a failure, and --quiet
hides the warnings but not the failures. A directory that cannot be
searched is likewise only a warning unless that flag is given.

The manifest itself is left out by file identity, as gen and freshen
do, so no name or spelling of its path or of the base gets it reported.

Model: opus-5-5

README.md

@@ -40,7 +40,7 @@ tree by URL:
 bin/mfer gen .
 
 # Verify the files on disk against the manifest. Exits nonzero if any file
-# is missing or corrupted.
+# it lists is missing or corrupted; warns about files it does not list.
 bin/mfer check index.mf
 
 # Download and cryptographically verify a tree published over HTTP: mfer
@@ -251,6 +251,9 @@ are now tracked only in the [issues](https://git.eeqj.de/sneak/mfer/issues).
 - `mfer check` / `mfer check .`
     - verifies checksums of all files in manifest, displaying error and exiting
       nonzero if any files are missing or corrupted
+    - warns about each file under the base directory that the manifest does not
+      list, hidden files included; with `--no-extra-files` each one is a failure
+      instead
 - `mfer fetch https://example.com/stuff/`
     - fetches `/stuff/index.mf` and downloads all files listed in manifest,
       optionally resuming any that already exist locally, and assures