tests: document AFP entry id smoke coverage

Document the Linux AFP Entry ID smoke-test coverage after the endpoint was verified against the standard SYS volume paths.

The test exercises the WebSDK-documented NCP 0x23/0x0c AFP Get Entry ID From Path Name request by logging in through libncp, allocating a temporary directory handle for the volume root, and sending the relative path to the AFP endpoint.

Record the verified SYS:, SYS:PUBLIC, SYS:SYSTEM, and SYS:BURST cases, and describe the current stat-derived fallback Entry ID diagnostics so the result is not confused with persistent CNID/AppleDouble storage.

Keep the remaining AFP work tracked in TODO.md, including replacing the fallback with persistent CNID/directory-id mapping and extending tests when additional AFP subfunctions are implemented.

This is documentation-only and does not change AFP protocol behavior.

TODO.md

@@ -192,6 +192,9 @@ Current status:
 
 - `NCP 0x23` still returns invalid namespace for AFP calls that are not implemented yet.
 - `AFP Get Entry ID From Path Name` is the first implemented AFP subfunction when the optional Netatalk/libatalk backend is available.
+  Linux smoke coverage exists in `tests/linux/afp_entry_id_smoke` and has been
+  verified against `SYS:`, `SYS:PUBLIC`, `SYS:SYSTEM`, and `SYS:BURST` with
+  stat-derived fallback entry IDs.
 - The AFP dispatcher now decodes the WebSDK/NWAFP subfunction number in
   diagnostics so real client probes can be mapped to the corresponding AFP
   call before implementation work starts.
@@ -213,6 +216,9 @@ Follow-up:
   data/resource fork and Finder Info semantics.
 - Replace the temporary stat-derived AFP entry-id fallback with a persistent
   CNID/directory-id mapping once the libatalk/CNID backend is integrated.
+- Extend `afp_entry_id_smoke` or add follow-up tests once additional AFP
+  subfunctions are implemented, especially file information, Finder Info, and
+  resource-fork handling.
 
 ## Deferred / optional protocol work