Solve workspace popover anchor from runtime sidebar width

DISCLOSURE_TOP_PX, DISCLOSURE_LEFT_PX, and DISCLOSURE_WIDTH_PX
were three hardcoded numbers that drifted the moment anyone touched
the sidebar header layout: SHELL_INSET, header padding, picker row
geometry, or the workspace tile size. Replace them with
WorkspaceDisclosureAnchor::solve(sidebar_width) — a pure function
whose algebra is the geometric inverse of render_sidebar_header,
expressed over named layout constants (HEADER_PT, PICKER_BUTTON_SIZE,
etc.) the renderer already applies.

Picker resize now flows: snapshot → sidebar_width → solve → anchor →
render_workspace_disclosure. Three regression tests lock the default
(66, 98, 180), invariance of left/top under resize, and zero-width
clamp. cargo test ely_app --bin ely_app: 101 passed.

crates/ely_app/src/shell/chrome/mod.rs

@@ -25,7 +25,8 @@ pub(crate) use plugin_detail_view::render_plugin_detail_view;
 pub(crate) use settings_layout::render_settings_shell;
 pub(crate) use sidebar::{panel_bg, panel_shadow};
 pub(crate) use sidebar_header::{
-    render_sidebar_header, render_workspace_disclosure, render_workspace_disclosure_backdrop,
+    WorkspaceDisclosureAnchor, render_sidebar_header, render_workspace_disclosure,
+    render_workspace_disclosure_backdrop,
 };
 pub(crate) use split_pane::{
     pane_host_label, pane_url_is_secure, render_compact_split_canvas,