fix(chart): give a broomed compare pane to its order book

[guyverino]

What & why

The comparison broom collapses a follower chart to nothing but its order book: the engine hides the price axis, floors the plot at one pixel and gives the book the whole pane. Input never learned that. Hit testing derived the book's width from the Order Book toggle alone, so it kept the book a strip along the right edge — and four fifths of a pane that is entirely order book answered clicks as chart. Orders could be placed, moved, dragged or cancelled only in that strip; a click further left panned an invisible plot, opened the coin on Main, or collapsed the stack's fullscreen.

The width was derived twice because there was nowhere to read it from. There is now: chartdx::pane_layout returns both areas — plot and book, placement and size — and prepare draws them while the panel hit-tests them.

On a broomed pane every chart gesture is now refused, whatever the "separate control zones" setting says: there is no plot there to pan, zoom, open on Main or leave fullscreen from.

Notable decisions

• No exceptions on a broomed pane. The open-on-Main double click and the right-click fullscreen exit are gone there too, deliberately: that pane is an order book and nothing else. The setting hint says so in all three locales.
• Two further copies removed. chartdx/input.rs had its own split in plot_metrics_for and try_dblclick_to_main. Fixing them also fixes the double click that opened a coin over the book's own left part whenever the price axis sat outboard of it, and the wheel that zoomed against a width the engine does not draw when the book is disabled.
• One predicate for "chart, not book" (chart_gesture_pane_at) — figure drawing, figure hit testing and the order-cross gate each answered it separately before.
• Subscription follows the drawing, not the toggle. Broom mode forces the book on with the Order Book toggle cleared, but demand followed the toggle — so such a pane rendered an empty book and, with the whole pane now its trading zone, invited orders against no depth. orderbook_drawn() is now the one spelling; historical viewers stay excluded.
• The book's left edge was floored one pixel inside the pane by an expression that cannot return zero. In broom mode that both misplaced what is drawn and left a column of chart at the pane's edge.

Known limitations

• Not driven live: this repo's UI verification is the maintainer's, and FireTest was not run. The unit tests cover the geometry, not the gestures.
• render.rs still derives a few logical-pixel layouts (button reserves, paddings) of its own; converting them is a separate change.
• The AddToChart wheel still reserves a left PRICE_AXIS_W strip regardless of the axis position — pre-existing, untouched here.

How to verify

cargo build --workspace --all-targets --target x86_64-pc-windows-msvc
cargo clippy -p moon-ui-gpui --bin moonterminal --target x86_64-pc-windows-msvc --all-targets -- -D warnings
cargo test --workspace --target x86_64-pc-windows-msvc

• build: pass.
• tests: pass — 1385 + 263 + the rest of the workspace green, including 11 new chartdx::tests unit tests for the shared pane layout and a new theme_contract contract pinning all four hit-test readers to it.
• clippy: this tree carries large pre-existing debt and -D warnings exits non-zero on origin/main too. No NEW finding: the only reports inside touched files are geom.rs !(range > 0.0) (two, untouched lines) and a collapsible if in render_input.rs — all pre-existing, verified by content.
• FireTest: not run (this repo's convention here).

By hand, on a comparison tab with the broom on: a left click anywhere on a follower pane places/moves orders instead of opening the coin on Main, the wheel over it scrolls the stack rather than zooming, and with the window's Order Book toggle off its book still has depth.