feat!: restore the egui GUI; withdraw the Dewey port

BREAKING CHANGE: deweygui is no longer a dependency, the `dewey-gui`
feature and `simonlib::gui_dewey` are removed, and `gui --script` speaks
simon's own goto/assert/capture vocabulary again rather than Dewey's JSON
agent protocol. `simonlib::gui::headless` and `gui::app::SiliconMonitorApp`
are back. MSRV returns to 1.70; Dewey's edition 2024 was the only thing
requiring 1.85.

4.0.0 replaced a ~10,000-line egui application with a ~2,100-line Dewey
one, and 4.0.1 through 4.0.4 spent four releases trying to make the
replacement presentable. Running the two side by side settles it: the
original shows nine metric cards including live GPU temperatures, six
sparkline charts with axes and min/max, a threshold legend, an uptime and
task status line, an AI ask bar and JSON/CSV export -- and it detects
three NVIDIA GPUs on the machine where the port reported "Accelerators:
unavailable -- Failed to initialize COM".

So the port was not merely less finished to look at; it was reading less
hardware. Neither fact reached the tests. They asserted that every tab
emitted named nodes, which the port did faithfully while showing a
fraction of what it replaced -- a contract that measured the shape of the
output and never its substance.

Restores src/gui/{app,widgets,theme,profile_tab,headless,mod}.rs, the
main.rs wiring, examples/gui.rs and the original contract tests from
927ffaa^. Reverts the three files whose 4.0.0 clippy fixes needed Rust
1.74+ (io::Error::other, is_none_or, repeat_n): with 1.70 declared again
clippy does not ask for those idioms, and asking for them would have
raised the MSRV by the back door.

Docs, ROADMAP and the CI comment go back to naming eframe.

The 30-second headless deadline and per-tab settle predicate come back
with it. Avoiding that machinery was the argument for the migration and
is the price of the reversal; CHANGELOG records it as a known issue
rather than leaving it to be rediscovered.

846 tests, 0 failures. clippy -D warnings clean at MSRV 1.70.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>