fix(doctests): ship a 256x256 placeholder icon, per the LGX icon contract

nix-bundle-lgx 6fb5d5b ("feat: add requirement for icons with manifest
0.4.0") validates the packaged icon straight out of the PNG IHDR: exactly
256x256, required for `type == "ui_qml"`. The tutorials embed a base64
placeholder that is 64x64, so `nix build '.#lgx'` now fails for calc_ui:

    ERROR: icon does not match the Logos icon standard.
      expected: PNG, exactly 256x256
      actual:   PNG, 64x64

That one rejection is the whole of the red run (208 passed, 3 failed):
`result-lgx/` is never created, so `Install the UI plugin` fails on a
literal `result-lgx/*.lgx`, and basecamp then launches without the plugin
("No clickable element found with text 'calc_ui'").

Regenerate the placeholder at 256x256, keeping the same solid #2ECC71. It
is now a 1-bit palette PNG rather than RGBA, which makes the embedded blob
shorter than the one it replaces (280 -> 144 base64 chars, 209 -> 106
bytes) despite the 16x pixel count.

The C++ UI tutorial carried the same 64x64 blob and was passing only
because its chain never reaches an `.#lgx` step — fixed here too, before
it surfaces.

Verified by extracting bundle.sh's validator verbatim and running the
tutorials' own `- run:` command through it: the old icon is rejected with
the exact CI error text, the new one passes.

outputs/*.md are hand-maintained (they carry deliberate /0.2.0 flake pins
that the generator does not emit), so the mirrored edits there are by hand
rather than regenerated.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

outputs/tutorial-cpp-ui-app.md

@@ -137,12 +137,12 @@ Replace the template contents with your plugin's details:
 }
 ```
 
-Create the icon directory and add a placeholder icon (displayed in the `logos-basecamp` sidebar when the module is loaded):
+Create the icon directory and add a placeholder icon — a PNG that is exactly 256×256, the only size LGX packaging accepts (displayed in the `logos-basecamp` sidebar when the module is loaded):
 
 ```bash
 mkdir -p icons
-# Copy any PNG here — or generate a 64×64 placeholder:
-echo "iVBORw0KGgoAAAANSUhEUgAAAEAAAABACAYAAACqaXHeAAAAmElEQVR4nO3QMREAIBDAsFeEN3ziCWRkoEP2XmedfX82OkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAO0BN/SiO/PatoIAAAAASUVORK5CYII=" | base64 -d > icons/calc.png
+# Copy any PNG here — or generate a 256×256 placeholder:
+echo "iVBORw0KGgoAAAANSUhEUgAAAQAAAAEAAQMAAABmvDolAAAABlBMVEUuzHEuzHEVOa2oAAAAH0lEQVR42u3BAQ0AAADCoPdPbQ43oAAAAAAAAAAAvg0hAAABYOSdlwAAAABJRU5ErkJggg==" | base64 -d > icons/calc.png
 ```
 
 Key fields:

outputs/tutorial-qml-ui-app.md

@@ -102,12 +102,12 @@ Replace the template contents with your plugin's details. The template may gener
 }
 ```
 
-Create the icon directory and add a placeholder icon. The icon is displayed in the `logos-basecamp` sidebar when the module is loaded:
+Create the icon directory and add a placeholder icon. It must be a PNG that is exactly 256×256 — LGX packaging rejects any other size. The icon is displayed in the `logos-basecamp` sidebar when the module is loaded:
 
 ```bash
 mkdir -p icons
-# Copy any PNG here — or generate a 64×64 placeholder:
-echo "iVBORw0KGgoAAAANSUhEUgAAAEAAAABACAYAAACqaXHeAAAAmElEQVR4nO3QMREAIBDAsFeEN3ziCWRkoEP2XmedfX82OkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAO0BN/SiO/PatoIAAAAASUVORK5CYII=" | base64 -d > icons/calc.png
+# Copy any PNG here — or generate a 256×256 placeholder:
+echo "iVBORw0KGgoAAAANSUhEUgAAAQAAAAEAAQMAAABmvDolAAAABlBMVEUuzHEuzHEVOa2oAAAAH0lEQVR42u3BAQ0AAADCoPdPbQ43oAAAAAAAAAAAvg0hAAABYOSdlwAAAABJRU5ErkJggg==" | base64 -d > icons/calc.png
 ```
 
 The `view` field tells the host which QML file to load for the UI. The `dependencies` field tells the host to load `calc_module` before showing your UI.

tests/tutorial-cpp-ui-app.test.yaml

@@ -144,13 +144,13 @@ sections:
               }
             }
         post_text: |
-          Create the icon directory and add a placeholder icon (displayed in the `logos-basecamp` sidebar when the module is loaded):
+          Create the icon directory and add a placeholder icon — a PNG that is exactly 256×256, the only size LGX packaging accepts (displayed in the `logos-basecamp` sidebar when the module is loaded):
 
-      - run: "mkdir -p icons && echo 'iVBORw0KGgoAAAANSUhEUgAAAEAAAABACAYAAACqaXHeAAAAmElEQVR4nO3QMREAIBDAsFeEN3ziCWRkoEP2XmedfX82OkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAO0BN/SiO/PatoIAAAAASUVORK5CYII=' | base64 -d > icons/calc.png"
+      - run: "mkdir -p icons && echo 'iVBORw0KGgoAAAANSUhEUgAAAQAAAAEAAQMAAABmvDolAAAABlBMVEUuzHEuzHEVOa2oAAAAH0lEQVR42u3BAQ0AAADCoPdPbQ43oAAAAAAAAAAAvg0hAAABYOSdlwAAAABJRU5ErkJggg==' | base64 -d > icons/calc.png"
         code_block: |
           mkdir -p icons
-          # Copy any PNG here — or generate a 64×64 placeholder:
-          echo "iVBORw0KGgoAAAANSUhEUgAAAEAAAABACAYAAACqaXHeAAAAmElEQVR4nO3QMREAIBDAsFeEN3ziCWRkoEP2XmedfX82OkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAO0BN/SiO/PatoIAAAAASUVORK5CYII=" | base64 -d > icons/calc.png
+          # Copy any PNG here — or generate a 256×256 placeholder:
+          echo "iVBORw0KGgoAAAANSUhEUgAAAQAAAAEAAQMAAABmvDolAAAABlBMVEUuzHEuzHEVOa2oAAAAH0lEQVR42u3BAQ0AAADCoPdPbQ43oAAAAAAAAAAAvg0hAAABYOSdlwAAAABJRU5ErkJggg==" | base64 -d > icons/calc.png
         post_text: |
           Key fields:
 

tests/tutorial-qml-ui-app.test.yaml

@@ -103,13 +103,13 @@ sections:
               }
             }
         post_text: |
-          Create the icon directory and add a placeholder icon. The icon is displayed in the `logos-basecamp` sidebar when the module is loaded:
+          Create the icon directory and add a placeholder icon. It must be a PNG that is exactly 256×256 — LGX packaging rejects any other size. The icon is displayed in the `logos-basecamp` sidebar when the module is loaded:
 
-      - run: "mkdir -p icons && echo 'iVBORw0KGgoAAAANSUhEUgAAAEAAAABACAYAAACqaXHeAAAAmElEQVR4nO3QMREAIBDAsFeEN3ziCWRkoEP2XmedfX82OkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAO0BN/SiO/PatoIAAAAASUVORK5CYII=' | base64 -d > icons/calc.png"
+      - run: "mkdir -p icons && echo 'iVBORw0KGgoAAAANSUhEUgAAAQAAAAEAAQMAAABmvDolAAAABlBMVEUuzHEuzHEVOa2oAAAAH0lEQVR42u3BAQ0AAADCoPdPbQ43oAAAAAAAAAAAvg0hAAABYOSdlwAAAABJRU5ErkJggg==' | base64 -d > icons/calc.png"
         code_block: |
           mkdir -p icons
-          # Copy any PNG here — or generate a 64×64 placeholder:
-          echo "iVBORw0KGgoAAAANSUhEUgAAAEAAAABACAYAAACqaXHeAAAAmElEQVR4nO3QMREAIBDAsFeEN3ziCWRkoEP2XmedfX82OkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAK0BOkBrgA7QGqADtAboAO0BN/SiO/PatoIAAAAASUVORK5CYII=" | base64 -d > icons/calc.png
+          # Copy any PNG here — or generate a 256×256 placeholder:
+          echo "iVBORw0KGgoAAAANSUhEUgAAAQAAAAEAAQMAAABmvDolAAAABlBMVEUuzHEuzHEVOa2oAAAAH0lEQVR42u3BAQ0AAADCoPdPbQ43oAAAAAAAAAAAvg0hAAABYOSdlwAAAABJRU5ErkJggg==" | base64 -d > icons/calc.png
         post_text: |
           The `view` field tells the host which QML file to load for the UI. The `dependencies` field tells the host to load `calc_module` before showing your UI.