NS-21: fix GdUnit CI path casing; preflight and README note

Linux res:// paths are case-sensitive; gdunit4 breaks, gdUnit4 is correct.

.github/workflows/gdscript.yml

@@ -23,6 +23,9 @@ on:
 jobs:
   lint_and_test:
     runs-on: ubuntu-latest
+    # Folder is addons/gdUnit4 (capital U). Lowercase gdunit4 fails on Linux (case-sensitive res://).
+    env:
+      GDUNIT_CMDTOOL: res://addons/gdUnit4/bin/GdUnitCmdTool.gd
     steps:
       - uses: actions/checkout@v4
 
@@ -58,7 +61,16 @@ jobs:
         working-directory: client
         run: ~/godot/Godot_v4.6-stable_linux.x86_64 --headless --import --path . --quit-after 10
 
+      - name: Verify GDUnit path (Linux res:// is case-sensitive)
+        working-directory: client
+        run: |
+          test -f addons/gdUnit4/bin/GdUnitCmdTool.gd || {
+            echo "::error::Missing GdUnit CLI script. Use addons/gdUnit4 (capital U), not gdunit4."
+            ls -la addons/ || true
+            exit 1
+          }
+
       - name: GDUnit4 (headless)
         working-directory: client
         run: |
-          ~/godot/Godot_v4.6-stable_linux.x86_64 --headless --path . -s res://addons/gdUnit4/bin/GdUnitCmdTool.gd --ignoreHeadlessMode -a res://test
+          ~/godot/Godot_v4.6-stable_linux.x86_64 --headless --path . -s "$GDUNIT_CMDTOOL" --ignoreHeadlessMode -a res://test

client/README.md

@@ -98,6 +98,8 @@ godot --headless --import --path . --quit-after 8
 godot --headless --path . -s res://addons/gdUnit4/bin/GdUnitCmdTool.gd --ignoreHeadlessMode -a res://test
 ```
 
+On **Linux** (including GitHub Actions), the path must be **`gdUnit4`** with a capital **`U`**; `gdunit4` does not resolve.
+
 **Scope:** Unit tests cover **`player.gd`**, **`position_authority_client.gd`**, and **`ground_pick.gd`** (walkable collider check). **`main.gd`**, full **`_input` / ray pick** flows, and scene wiring are **not** automated here—use manual checks above.
 
 **Reports:** GdUnit writes under **`reports/`** (gitignored); ignore locally generated HTML/XML when committing.