fix(prompt): drop superseded background renders via generation stamp

[plttn]

Summary

• After cd-ing out of a git repo, the prompt kept showing the old repo's git status and right-prompt items (node/rustc version, etc.), sometimes for a long, unpredictable time before clearing on its own.
• Root cause: background render jobs could finish out of order. The "kill the previous job" check only catches a job that has already started writing its scratch file, but fish -c takes real time to start up, so two renders could be in flight at once. Whichever finished last won, not whichever was dispatched last — and an in-repo render (git status forks, toolchain-version items) is reliably slower than a plain directory's render, so the stale one usually finished last.
• Fix: stamp each dispatched render with a generation number (_tide_render_gen, a plain in-memory global, no universal variables involved) and drop any result whose stamp doesn't match the latest dispatch, so a superseded render can no longer overwrite a fresher one.

Test plan

• mise run lint
• mise run test (full littlecheck suite, including tests/fish_prompt.test.fish and tests/fish_prompt_transient.test.fish)
• Isolated logic test reproducing the exact race (stale generation landing after a fresh one) confirms the stale result is dropped and prompt_var stays correct
• Manual: cd into a git repo with toolchain files, then cd .., confirm the prompt updates promptly and stays correct across repeats