chore: re-pin logos-protocol for Codec<std::optional<T>> (#178)

This repo declares logos-cpp-sdk.inputs.logos-protocol.follows and
logos-qt-sdk.inputs.logos-protocol.follows (flake.nix:12,17), so ITS protocol
pin is authoritative for the whole closure -- cpp-sdk's own pin of the
protocol carrying Codec<std::optional<T>> (logos-protocol#37) is overridden
and never reaches a module build.

Result: the generator correctly emits Codec<std::optional<std::string>> for
an optional record field and the module then fails to compile it:

  error: implicit instantiation of undefined template
         'logos::detail::Codec<std::optional<std::string>>'

#177 bumped cpp-sdk, rust-sdk and qt-sdk but not protocol, and the follows
made that omission invisible -- every check passed, because nothing in this
repo's own check set declares an optional field.

logos-protocol 4ee85b2 -> 72754ab. All five checks build.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

flake.lock

@@ -25335,11 +25335,11 @@
         ]
       },
       "locked": {
-        "lastModified": 1785352565,
-        "narHash": "sha256-i5ynBtNk6Qe+YSHsa+O8DjnLpHXDVqyLGPxBAImipD8=",
+        "lastModified": 1785470728,
+        "narHash": "sha256-K6rr5ycCWctvtUnktskjRlXHN3GuI7FEbz8/FX+pVY8=",
         "owner": "logos-co",
         "repo": "logos-protocol",
-        "rev": "4ee85b26a65e331821823f6435135c088a1bf447",
+        "rev": "72754ab9b2ea8a43a3f04c5bdf1411673f3f489e",
         "type": "github"
       },
       "original": {