fix(custom-model): keep the in-use picker honest about down endpoints and exited panes

Review round on PR #757. The route turns EVERY upstream failure into
isLlamaSwap: false, so the old line silently promised an unreachable
endpoint was merely mute, and the live-session check missed a null pid
(the PTY exited, the same test keep-warm applies).

- Require pid !== null in _getCustomModelInUseBySession, pinned by a
  'process exited' case in the marks-nothing table
- Reword #customModelPickUnknown to claim only that nothing was
  reported, true for a down endpoint too; drop the 'stays quiet for a
  failed probe' promise from changeset, docs and comments
- architecture-invariants: add the In-use tier, both pin fences
  (customModel not displayModel; stopped/null-pid/paneExit never
  count) and the unknown-line rule
- Mirror the wording through i18n (key + zh-CN), index.html fixture,
  styles.css comment, and the run-menu tests

.changeset/custom-model-picker-in-use.md

@@ -2,4 +2,4 @@
 'aicodeman': patch
 ---
 
-The "Choose a model" picker now says which model is actually in use on an endpoint that cannot report it. Only llama-swap answers that question, so behind a gateway such as Bifrost (and behind a bare llama.cpp, Ollama or vLLM) nothing was marked and the list fell back to plain discovery order, apart from the one CLI that happened to have launched there before. A live session whose own injected endpoint is that host now promotes its model as **"In use by a session"**, under its own tag and behind llama-swap's own "Currently loaded" answer, reading only what Codeman injected rather than text scraped off a terminal, and never a stopped session. When genuinely nothing is known the picker says so instead of leaving an unmarked wall, and it stays quiet for an endpoint that merely failed to answer. Pinned by test/custom-model-run-menu-ui.test.ts.
+The "Choose a model" picker now says which model is actually in use on an endpoint that cannot report it. Only llama-swap answers that question, so behind a gateway such as Bifrost (and behind a bare llama.cpp, Ollama or vLLM) nothing was marked and the list fell back to plain discovery order, apart from the one CLI that happened to have launched there before. A live session whose own injected endpoint is that host now promotes its model as **"In use by a session"**, under its own tag and behind llama-swap's own "Currently loaded" answer, reading only what Codeman injected rather than text scraped off a terminal, and never a stopped or exited session. When nothing is known the picker says so instead of leaving an unmarked wall; its wording claims only that nothing was reported, which stays true for an endpoint that is down, since the route answers every probe failure as `isLlamaSwap: false`. Pinned by test/custom-model-run-menu-ui.test.ts.