nwconn: persist additional AFP metadata attributes

Extend the conservative AFP 2.0 Set File Information smoke path to accept the metadata-only System and Backup file attribute bits alongside the already-supported Finder Invisible bit.

The WebSDK/NWAFP Set File Information request uses the file Attributes bitmap to pass a set/clear attribute word. Netatalk stores several AFP file attributes in AppleDouble metadata while computing open-fork state dynamically and leaving enforcement-sensitive bits to the file/fork paths. Mirror only the low-risk mars_nwe subset here: persist Invisible, System, and Backup in the private org.mars-nwe.afp.attributes xattr, and keep NoWrite, NoRename, NoDelete, NoCopy, data-fork-open, resource-fork-open, timestamps, resource forks, and Entry-ID-only write semantics rejected until they have deliberate enforcement and backend design.

The xattr payload remains versioned and unchanged. Reads now expose the three supported metadata bits through Get/Scan File Information, and writes preserve the existing set/clear semantics over the supported mask.

Update the Linux Set File Information smoke helper with --system/--clear-system and --backup/--clear-backup options, while keeping the smoke-suite default unchanged. Document the expected Linux xattr forms 0x01000004 and 0x01000040 for those optional probes.

Tests: git diff --check; gcc -Iinclude -I/mnt/data/stubs -fsyntax-only tests/linux/afp_set_file_info_smoke.c.

tests/linux/README.md

@@ -449,8 +449,9 @@ NCP 0x2222/35/16  AFP 2.0 Set File Information
 
 The helper exercises two deliberately narrow write-safe AFP metadata subsets:
 the file FinderInfo bitmap (`0x0020`) and the file Attributes bitmap (`0x0001`)
-restricted to the Finder Invisible bit.  It sends path-backed raw `VOL:`-style
-requests, writes the 32-byte FinderInfo block to mars_nwe's private
+restricted to metadata-only file flags: Finder Invisible, System, and Backup.
+It sends path-backed raw `VOL:`-style requests, writes the 32-byte FinderInfo
+block to mars_nwe's private
 `org.mars-nwe.afp.finder-info` metadata key, writes the narrow AFP attribute word
 to `org.mars-nwe.afp.attributes`, and immediately verifies the updates through
 AFP 2.0 Get File Information.  On Linux the source-level `org.mars-nwe.afp.*` name is stored via the
@@ -522,10 +523,30 @@ user.org.mars-nwe.afp.attributes=0x01000001
 ```
 
 Use `--clear-invisible --attributes-only` to clear that bit; the same xattr then
-stores `0x01000000`.  All other Set File Information bitmap bits and all other
-AFP attribute bits are intentionally rejected for now.  That keeps timestamp,
-DOS/NetWare mode-bit mapping, resource-fork, and Entry-ID-only write semantics
-out of this metadata-only smoke path.
+stores `0x01000000`.  The same helper can exercise the two additional
+metadata-only file attribute bits that Netatalk treats as stored AFP file
+attributes and that do not require immediate Unix mode-bit or DOS/NetWare
+attribute mutation:
+
+```sh
+./tests/linux/afp_set_file_info_smoke \
+  -S MARS -U SUPERVISOR -P secret \
+  --attributes-only --system \
+  SYS:PUBLIC/pmdflts.ini
+
+./tests/linux/afp_set_file_info_smoke \
+  -S MARS -U SUPERVISOR -P secret \
+  --attributes-only --backup \
+  SYS:PUBLIC/pmdflts.ini
+```
+
+For System the xattr value stores `0x01000004`; for Backup it stores
+`0x01000040`.  Use `--clear-system` and `--clear-backup` to remove those bits.
+All other Set File Information bitmap bits and AFP attribute bits, including
+NoWrite, NoRename, NoDelete, NoCopy, and the computed data/resource-fork-open
+flags, are intentionally rejected for now.  That keeps timestamp,
+DOS/NetWare mode-bit mapping, enforcement, resource-fork, and Entry-ID-only
+write semantics out of this metadata-only smoke path.
 
 If the server was built without the optional Netatalk/libatalk backend, use
 `--allow-invalid-namespace` for the expected negative test.  Use