fix(docs): target the RTD uv environment

Install documentation requirements with the Python executable from the Read the Docs virtual environment so the subsequent uv run can find Sphinx.

Coding-Agent: Codex
Codex-Version: codex-cli 0.144.6
Model: gpt-5.6-sol
Reasoning-Effort: xhigh

Author: njzjz-bot

.readthedocs.yml

@@ -3,6 +3,10 @@ build:
   os: ubuntu-26.04
   tools:
     python: "3.11"
+  jobs:
+    install:
+      # Install into the same uv environment used by the later build command.
+      - uv pip install --python "$READTHEDOCS_VIRTUALENV_PATH/bin/python" -r doc/requirements.txt
 python:
   install:
     - method: uv