[Linux] Retain software renderer frame state until finalization - #196546

A realized software renderer can still present a frame after GTK destroys its widget while a strong reference keeps the instance alive. dispose() currently clears task_runner; present_layers() then reaches fl_task_runner_stop_wait(nullptr) and crashes in g_cond_signal().

Keep the task runner and frame mutex until finalize(), alongside the existing compositor and surface lifetime. Add a realized-widget regression; the existing unrealized-widget test returns before reaching this path.

Fixes #196545.

Related: #193533 handles OpenGL/Subsurface lifetime. This change is limited to the software renderer.

Validation

• The new regression produces SIGSEGV on unmodified master 215858ff8a1e51fb5481c7c7cc4c6d4be54b8ae0; the symbolized stack shows fl_task_runner_stop_wait(self=0x0).
• With the fix: 100 consecutive regression iterations and all 862 Linux engine unit tests pass under D-Bus/Xvfb on Ubuntu 24.04 / GTK 3.24.41.
• The engine's bundled clang-format and pre-push formatting checks pass.

ninja -C engine/src/out/host_debug_unopt flutter_linux_unittests
dbus-run-session -- xvfb-run -a engine/src/out/host_debug_unopt/flutter_linux_unittests
dbus-run-session -- xvfb-run -a engine/src/out/host_debug_unopt/flutter_linux_unittests \
  --gtest_filter=FlViewRendererSoftwareTest.PresentLayersAfterRealizedDestroy \
  --gtest_repeat=100 --gtest_break_on_failure

Prepared with Codex assistance. Kept as a draft for contributor review under the AI contribution guidelines.

Pre-launch Checklist

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• I listed at least one issue that this PR fixes in the description above.
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• All existing and new tests are passing.