Closes the last operational gap on the designer pipeline (issue #69).
The fork's 0.4.x tool surface was token-CRUD only, so every designer
dispatch hit the "no shape primitives" diagnostic and aborted without
producing Penpot artifacts. This lands the minimum set that makes the
`design-implement.md` skill's happy path callable end-to-end:

Read / navigate (thin RPC wrappers, no DB pool):
- list_teams()
- list_projects(team_id)
- list_files(project_id)

Canvas primitives (single `update-file` change-op each):
- create_page(file_id, name) — emits add-page
- create_frame(file_id, page_id, name, x, y, w, h, fill_color?, parent_id?)
  — emits add-obj with type=frame, axis-aligned envelope (selrect +
  points + identity transform computed from x/y/w/h)
- create_text(file_id, page_id, parent_id, content, x, y, w, h, ...)
  — emits add-obj with type=text and Penpot's nested
  `root → paragraph-set → paragraph → text-node` content tree.
  Single-paragraph plain text only; styled / multi-paragraph content
  is deferred.

Fix — apply_changes sends `vern` + camelCase sessionId:

  The token write tools landed in 0.3 silently broke against live
  Penpot on any server build that required a vern field on
  update-file. Unit tests mocked above the HTTP layer so the gap
  never surfaced. Fixed the client here — transit-json decode was
  already fixed on 0.4's access-token refactor (`Accept:
  application/json`), the remaining key-case mismatch
  (`session-id` vs `sessionId`) is fixed in the same spot.

Tests:

  12 unit tests in tests/test_canvas.py cover:
  - read tools pass correct param keys (teamId, projectId — camelCase)
  - create_page emits a single add-page change with a fresh UUID each
    call, returned id matches the one sent
  - create_frame emits add-obj at page root by default, nests under
    parent_id when passed, omits fill when fill_color is None
  - create_frame geometry envelope is consistent (selrect bounds,
    4-corner points, identity transform) — Penpot rejects mismatches
  - create_text wraps content in the root/para/leaf tree and sets
    obj.growsType = "auto-width"
  - create_text defaults match upstream conventions
    (sourcesanspro / 14 / 400 / left / #000000)
  - obj.name is sliced to 40 chars so the layers panel stays readable

  Tool functions are module-level (not buried in a register() closure
  like tokens.py) so tests import them directly without FastMCP's
  private _tools attr lookup — keeps the tests future-proof against
  the next mcp library rename.

Live-verified end-to-end against the existing file from #55
(689d7fa4-f94b-81d4-8007-e39c5c82f66c): a `canvas-probe` page with
an 800×600 frame filled `#1A1B26` and a centred `HELLO` text at
Fira Code 40/700 `#C0CAF5` — exactly the AC on #62.

Not shipped here (tracked separately):
- create_file — richer param surface than the other writes
- export_frame_png — Penpot renders PNGs client-side via wasm on
  recent builds; no stable server-side export RPC to wrap

@@ -0,0 +161,4 @@
+        x=x,
+        y=y,
+        width=width,
+        height=height,

@@ -0,0 +240,4 @@
+            FILE_ID, PAGE_ID, ROOT_FRAME_ID,
+            content=long_text, x=0, y=0, width=100, height=20,
+        )
+    assert calls[0][1][0]["obj"]["name"] == "x" * 40  # sliced to 40 chars

@@ -0,0 +188,4 @@
+                                "text": content,
+                                "fontFamily": font_family,
+                                "fontSize": str(font_size),
+                                "fontWeight": font_weight,

@@ -0,0 +193,4 @@
+                                "textAlign": text_align,
+                                "textDecoration": "none",
+                                "lineHeight": 1.2,
+                                "letterSpacing": "0",

@@ -0,0 +215,4 @@
+    assert result["shape_id"] == change["id"]
+
+
+@pytest.mark.asyncio

@@ -6,0 +35,4 @@
+Module-level tool functions (not buried in a `register()` closure)
+so the unit tests in `tests/test_canvas.py` can import and invoke
+them directly — avoids the `mcp._tools` private-attr lookup the
+tokens tests used, which broke with the `mcp` library's internal