fix(sidebar): hoist _sessionAttentionState to fix ReferenceError crash (#3696) + scope-undef prevention gate (#3698)

* fix(sidebar): hoist _sessionAttentionState to top-level scope (#3696)

_sessionAttentionState was declared inside renderSessionListFromCache() and
relied on function hoisting, but the top-level function _sidebarRowHasVisible
Messages (reached via renderSessionListFromCache -> _partitionSidebarSessionRows)
called it bare. Hoisting is scoped to the enclosing function, so every sidebar
cache-render threw 'ReferenceError: _sessionAttentionState is not defined' and
the session list went blank. Regressed in #3672 (v0.51.269) when _sidebarRow
HasVisibleMessages was extracted to top level.

Fix: move _sessionAttentionState to top-level scope (it is pure — only uses its
arg plus the i18n global t), so both the visibility predicate and the nested
per-row renderer can reach it.

Prevention (the durable half): add scripts/scope_undef_gate.py — models the
classic-<script> shared global scope (union of all static files' top-level
symbols) and runs ESLint no-undef per file, flagging a function defined nested
but called from a sibling scope. Wired into CI (.github/workflows/tests.yml lint
job) alongside the existing no-const-assign runtime gate, plus an in-suite test
(test_static_js_scope_undef.py) and a focused structural regression test
(test_issue3696_session_attention_scope.py). RED/GREEN-validated against the
broken tree.

* fix(streaming): thread source param into stale-stream bailout; tighten scope gate

Opus review of #3698 found the new scope_undef_gate's 'source' allowlist entry
was masking a real same-class bug: _bailOutOfTerminalEventsFromStaleStream
(declared inside attachLiveStream, params activeSid/streamId/uploaded/options)
called _closeSource(source) against a 'source' not in its lexical scope. All 5
call sites are inside _wireSSE(source), but JS scope is lexical not dynamic, so
the helper would throw ReferenceError: source is not defined on the stale-stream
terminal-event path (user back in an active session whose old stream finalizes
late).

Fix: thread source as an explicit parameter (declaration + all 5 call sites),
the same make-the-dependency-explicit fix as #3696 — and REMOVE the 'source'
allowlist entry so the gate stays gated against that name (it now passes because
the bug is fixed, not because it's allowlisted). Added the documented
false-negative classes from Opus's review to the gate docstring (name-collision
shadowing, destructuring-regex gap, exposure escape hatches, name-keyed
allowlist) and a focused regression test.

This is the prevention gate catching a real latent bug on its first outing.

---------

Co-authored-by: nesquena-hermes <nesquena-hermes@users.noreply.github.com>

CHANGELOG.md

@@ -3,6 +3,13 @@
 
 ## [Unreleased]
 
+### Fixed
+- **Sidebar no longer crashes with `ReferenceError: _sessionAttentionState is not defined`.** The session-attention helper was declared *inside* `renderSessionListFromCache()` and relied on function hoisting, but the top-level `_sidebarRowHasVisibleMessages` (reached via `renderSessionListFromCache` → `_partitionSidebarSessionRows`) called it bare — and hoisting is scoped to the enclosing function, so every sidebar cache-render threw and the session list went blank. `_sessionAttentionState` is now a top-level function reachable by both call sites. Regressed in #3672 (v0.51.269). (#3696)
+- **Stale-stream terminal events no longer risk a `ReferenceError: source is not defined`.** `_bailOutOfTerminalEventsFromStaleStream` (declared inside `attachLiveStream`) called `_closeSource(source)` against a `source` that was not in its lexical scope — it would have thrown on the late-finalizing-stream path when the user is back in an active session. `source` is now threaded as an explicit parameter. Found by the new scope gate below during review. (#3696)
+
+### Internal
+- **New static-JS scope/undefined-reference gate (`scripts/scope_undef_gate.py`).** Models the WebUI's classic-`<script>` shared global scope and runs ESLint `no-undef` per file, flagging a function that is defined only *nested* but called from a sibling/top-level scope — the brick class behind #3696 that `node --check`, source-presence tests, and the existing `no-const-assign` runtime gate all miss. Wired into the CI `lint` job alongside the `no-const-assign`/`no-import-assign` runtime gate, with an in-suite test (`tests/test_static_js_scope_undef.py`) and a focused structural regression test (`tests/test_issue3696_session_attention_scope.py`). (#3696)
+
 ## [v0.51.288] — 2026-06-06 — Release JD (stage-r24 — collapsible approval card)
 
 ### Added