nwatalk: cache AFP fallback entry ids in xattrs

The AFP smoke endpoints can now read mars_nwe-owned entry ids from the versioned org.mars-nwe.afp.entry-id xattr, but a newly discovered file still had to fall back to the temporary stat-derived id on every request until a real CNID allocator exists.

Preserve the existing WebSDK/NWAFP response semantics while making that fallback sticky: when Get Entry ID, Get File Information, or Scan File Information has no mars_nwe xattr and no Netatalk/libatalk AppleDouble/CNID id, derive the existing compatibility id and cache it through nwatalk_set_entry_id().  The first request still logs fallback so diagnostics remain honest about the id origin; subsequent requests should read the xattr directly and avoid re-entering the stat fallback path.

Keep the write narrowly scoped to mars_nwe's private AFP metadata namespace.  The payload is versioned, big-endian, and stored through the nwxattr helper, so Linux persists it as user.org.mars-nwe.afp.entry-id while source-level code continues to use the Netatalk-style org.mars-nwe.afp.entry-id name.  This does not implement CNID allocation, parent-id lookup, entry-id-only resolution, FinderInfo mutation beyond the existing smoke path, or resource-fork semantics.

Tests:

- git diff --check

- cmake --build build-xattr-off --target nwconn with ENABLE_NETATALK_LIBATALK=OFF

- cmake --build build-xattr-on --target nwconn with ENABLE_NETATALK_LIBATALK=ON against Netatalk 4.4.3 headers plus local link stubs

tests/linux/README.md

@@ -7,11 +7,13 @@ The tests use the ncpfs/libncp client library.  They are not built by default
 because they require the host ncpfs development headers/library and a running
 NetWare-compatible server.
 
-The AFP endpoints are intentionally conservative.  When persistent AFP entry
-ids become available, mars_nwe-owned ids are read from the versioned
-`org.mars-nwe.afp.entry-id` xattr before falling back to Netatalk/libatalk
-AppleDouble/CNID metadata and then the temporary stat-derived fallback.  The
-first AFP write smoke path is deliberately limited to the FinderInfo bitmap of
+The AFP endpoints are intentionally conservative.  mars_nwe-owned ids are read
+from the versioned `org.mars-nwe.afp.entry-id` xattr before falling back to
+Netatalk/libatalk AppleDouble/CNID metadata.  When neither source has an id yet,
+the existing stat-derived compatibility id is cached in that xattr so subsequent
+AFP probes can reuse the same mars_nwe-owned id instead of re-entering the
+temporary fallback path.  The first AFP write smoke path is deliberately limited
+to the FinderInfo bitmap of
 AFP 2.0 Set File Information; CNID allocation, DOS attribute mapping, resource
 fork writes, and data-fork writes remain separate write-safety work.  mars_nwe
 source uses Netatalk-style `org.mars-nwe.<domain>.*` xattr names; AFP
@@ -401,13 +403,29 @@ AFP 2.0 Set File Information: vol=0 request_vol=0 entry=0x00000000 mask=0x0020 p
 AFP 2.0 Get File Information: vol=0 entry=0x00000000 mask=0xffff path='SYS:PUBLIC/pmdflts.ini' reply_entry=0x23c8787d fallback
 ```
 
-The `fallback` marker on the verification Get File Information diagnostic still
-refers to the entry-id source: the returned entry id is derived from the current
-stat-backed fallback path because no persistent CNID or `org.mars-nwe.afp.entry-id`
-metadata was present.  It does not mean the FinderInfo write was ignored; the
+The `fallback` marker on the first verification Get File Information diagnostic
+still refers to the entry-id source: the returned entry id was derived from the
+stat-backed compatibility path because no CNID or mars_nwe entry-id xattr existed
+yet.  The server now caches that derived id in `org.mars-nwe.afp.entry-id`, so a
+second probe of the same file should reuse the xattr-backed id and normally omit
+the `fallback` marker.  It does not mean the FinderInfo write was ignored; the
 helper verifies the written FinderInfo through the follow-up Get File Information
 reply.
 
+Linux xattr checks for the FinderInfo and cached Entry ID look like this:
+
+```sh
+getfattr -n user.org.mars-nwe.afp.finder-info -e hex /var/mars_nwe/SYS/public/pmdflts.ini
+getfattr -n user.org.mars-nwe.afp.entry-id -e hex /var/mars_nwe/SYS/public/pmdflts.ini
+```
+
+For the verified FinderInfo smoke run, the FinderInfo xattr starts with
+`TEXTMARS`:
+
+```text
+user.org.mars-nwe.afp.finder-info=0x544558544d415253000000000000000000000000000000000000000000000000
+```
+
 All other Set File Information bitmap bits are intentionally rejected for now.
 That keeps attribute, timestamp, DOS/NetWare mode-bit mapping, resource-fork,
 and Entry-ID-only write semantics out of this first metadata-only write smoke