refactor(gui_2): migrate CommsLogEntry consumers to direct field access

Phase 3: CommsLogEntry
Before: 3 .get('source_tier',...) sites + 1 half-measure in src/gui_2.py
After:  0
Delta:  -4 (expected: -5 per plan; the 5th site was app_controller.py:1930
        which returns None for missing source_tier and cannot be migrated
        without breaking test_append_tool_log_dict_keys)

Migrates the following CommsLogEntry-related sites in src/gui_2.py:

1. gui_2.py:1810 - cache filter source_tier (.get('source_tier', ''))
2. gui_2.py:1818 - cache filter source_tier (.get('source_tier', ''))
3. gui_2.py:5104 - render_comms_log_panel source_tier (.get('source_tier', 'main'))
4. gui_2.py:5106 - render_comms_log_panel ts (.get('ts', '00:00:00'))
5. gui_2.py:5107 - render_comms_log_panel direction (.get('direction', '??'))
6. gui_2.py:5110 - render_comms_log_panel model (.get('model', '?'))
7. gui_2.py:5802 - render_tool_calls_panel half-measure
        (subscript + 'in' check; entry['source_tier'] if 'source_tier' in entry else 'main')

All migrated via:
  ce = CommsLogEntry.from_dict(entry)
  ce.<field>           # direct attribute access

The dataclass default for source_tier is 'main', which preserves the
fallback behavior for sites that had 'main' as the default. For sites
with '' as the default (cache filters), the behavior change is benign
because both '' and 'main' fail to match any non-trivial agent prefix.

Notes:
- The 'kind' field is NOT migrated because it has a legacy 'type'
  fallback ('kind' OR 'type') that the dataclass default doesn't
  preserve.
- 'provider' and 'payload' are NOT on CommsLogEntry; they remain
  as entry.get(...) calls.
- src/app_controller.py:1930 is NOT migrated because its
  no-default behavior (returns None) is asserted by
  test_append_tool_log_dict_keys.

Tests: 16/16 pass (test_mma_agent_focus_phase1, test_comms_log_entry,
test_gui2_events).

src/gui_2.py

@@ -1807,7 +1807,7 @@ def render_main_interface(app: App) -> None:
   if app.is_viewing_prior_session: app._comms_log_cache = app.prior_session_entries
   else:
    log_raw = list(app._comms_log)
-   if app.ui_focus_agent: app._comms_log_cache = [e for e in log_raw if e.get("source_tier", "").startswith(app.ui_focus_agent)]
+   if app.ui_focus_agent: app._comms_log_cache = [e for e in log_raw if CommsLogEntry.from_dict(e).source_tier.startswith(app.ui_focus_agent)]
    else:                  app._comms_log_cache = log_raw
   app._comms_log_dirty = False
  
@@ -1815,7 +1815,7 @@ def render_main_interface(app: App) -> None:
   if app.is_viewing_prior_session: app._tool_log_cache = app.prior_tool_calls
   else:
    log_raw = list(app._tool_log)
-   if app.ui_focus_agent: app._tool_log_cache = [e for e in log_raw if e.get("source_tier", "").startswith(app.ui_focus_agent)]
+   if app.ui_focus_agent: app._tool_log_cache = [e for e in log_raw if CommsLogEntry.from_dict(e).source_tier.startswith(app.ui_focus_agent)]
    else:                  app._tool_log_cache = log_raw
   app._tool_log_dirty = False
  
@@ -5096,12 +5096,13 @@ def render_comms_history_panel(app: App) -> None:
    imgui.push_id(f"comms_entry_{i}")
   
    i_display = i + 1
-   ts        = entry.get("ts",        "00:00:00")
-   direction = entry.get("direction", "??")
+   ce        = CommsLogEntry.from_dict(entry)
+   ts        = ce.ts        or "00:00:00"
+   direction = ce.direction or "??"
    kind      = entry.get("kind", entry.get("type", "??"))
    provider  = entry.get("provider",    "?")
-   model     = entry.get("model",       "?")
-   tier      = entry.get("source_tier", "main")
+   model     = ce.model     or "?"
+   tier      = ce.source_tier
    payload   = entry.get("payload",     {})
    if not payload and kind not in ("request", "response", "tool_call", "tool_result"):
     payload = entry # legacy
@@ -5799,7 +5800,7 @@ def render_tool_calls_panel(app: App) -> None:
     app.show_windows["Text Viewer"] = True
 
    imgui.table_next_column()
-   imgui.text_colored(C_SUB(), f"[{entry['source_tier'] if 'source_tier' in entry else 'main'}]")
+   imgui.text_colored(C_SUB(), f"[{CommsLogEntry.from_dict(entry).source_tier}]")
 
    imgui.table_next_column()
    script_preview = script.replace("\n", " ")[:150]