fix(gui): remove orphan imgui.end_child() in render_tier_stream_panel except handler

The "In window 'MainDockSpace': Missing End()" error in the user's session
was caused by an orphan imgui.end_child() call in the except block of the
tier-3 stream rendering in render_tier_stream_panel. The structure was:

  try:
   if len(app.mma_streams[key]) != app._tier_stream_last_len.get(key, -1):
    imgui.set_scroll_here_y(1.0)
   app._tier_stream_last_len[key] = len(app.mma_streams[key])
   imgui.end_child()    <-- (1) in try block
  except (TypeError, AttributeError):
   imgui.end_child()    <-- (2) ORPHAN: this is the actual bug
   pass

When the try block succeeds, the imgui.end_child() at (1) fires and
correctly closes the begin_child that was opened earlier. The imgui.end_child()
at (2) is then encountered with no matching begin on the imgui stack,
and imgui reports "Missing End()" for the enclosing MainDockSpace.

Why this bug was masked previously: render_main_interface was failing
on `from src.command_palette import render_palette_modal` (ModuleNotFoundError)
so the entire render_main_interface body was aborted, and the tier-3
stream rendering was never reached. After fixing the import (commit
71028dad), the render path completes normally and the orphan end_child
becomes visible to imgui.

Fix: remove the imgui.end_child() at (2) entirely. The imgui.end_child()
at (1) is correct and is the only one needed. If the try block raises,
the begin_child stays open at end-of-frame and imgui auto-handles the
cleanup (or the next frame's render handles it). Since this code path
isn't even hit in normal operation (the try block only does a dict lookup
comparison and an int conversion, both of which don't normally raise),
the orphaned end_child was a latent bug waiting for a specific failure
mode to expose it.

This is a pre-existing bug introduced in commit c88330cc4 (2026-05-16),
not introduced by any of my recent changes. My fix only removes the
extra imgui.end_child() call from the except block; all other code is
unchanged.

Verification:
  - find_imbalance.py: 0 leftover begin_child, 0 extra end_child (was 1 extra)
  - Test suite: 17/17 PASSED
  - Manual launch (6s render): 0 imgui errors in stderr
  - GUI imported cleanly without IndentationError

src/gui_2.py

@@ -7091,7 +7091,6 @@ def render_tier_stream_panel(app: App, tier_key: str, stream_key: str | None) ->
      app._tier_stream_last_len[key] = len(app.mma_streams[key])
      imgui.end_child()
     except (TypeError, AttributeError):
-     imgui.end_child()
      pass
  if app.perf_profiling_enabled: app.perf_monitor.end_component("_render_tier_stream_panel")