Show context-menu separators only where the renderer draws them

[kdroidFilter]

Summary

• Spellcheck no longer replaces LocalContextMenuRepresentation. The hand-rolled Compose menu that existed only to paint a hairline for an empty-label sentinel is gone (187 lines), along with SpellcheckContextMenuSeparator and LocalSpellcheckMenuSeparator. That popup used hardcoded colors driven by isSystemInDarkTheme(), dropped keyboard navigation and a11y, and only engaged when the clicked word was misspelled — so the menu looked different between two right clicks in the same field.
• Renderers now declare the capability instead of consumers inferring it: LocalContextMenuDivider (nullable, default null) is provided by NativeContextMenuProvider (NucleusContextMenuDivider, drawn by NSMenu on macOS and by the Fluent / Adwaita / Breeze flyouts elsewhere) and by ProvideJewelSpellcheckMenu (Jewel's ContextMenuDivider). With Compose's own representation there is no divider to draw, so none is emitted. Apps wanting dividers outside Jewel already have a supported path: nativeContextMenu = true.
• spellcheckMenuSections / spellcheckContextMenuItems / NucleusSpellcheckInstaller.menuItems take a nullable separator; ContextMenuEntry.kt no longer imports the spellcheck package.
• NativeContextMenuRepresentation normalizes interpreted entries — trims leading/trailing separators, collapses runs, recurses into submenus — since menu items come from several independent contributors (field Cut/Copy/Paste, app extras, spellcheck).
• Fixes the Adwaita dark separator: it painted popover_shade_color (25 % black), so the rule came out darker than the menu surface. libadwaita uses separator { background: $border_color } with $border_color: color-mix(in srgb, currentColor var(--border-opacity), transparent) and --border-opacity: 15% — i.e. 15 % of the text color: white on a dark menu, RGB(0 0 6 / 80%) premultiplied to 12 % on a light one (was 7 %). Breeze and Fluent were already correct.
• apiDump refreshed. The removed symbols were never published (latest release is 2.4.4), so nothing is deprecated.

Test plan

• ./gradlew :nucleus-application:check :decorated-window-jewel:check — detekt, ktlint, apiCheck, tests
• ./gradlew :examples:jewel-demo:test :examples:jewel-demo:ktlintCheck
• New ContextMenuDividerCapabilityTest — no divider published by default, NucleusContextMenuDivider under the OS-looking provider, none when it is disabled
• New ContextMenuFlyoutSeparatorTest — Adwaita's 15 %-of-currentColor rule, and dark separators lighten / light separators darken the surface across Adwaita, Breeze and Fluent
• SpellcheckInstallerTest — no separator and no stand-in row when the renderer cannot draw one, for both placements
• ./gradlew :examples:tao-demo:compileKotlin :examples:nucleus-demo:compileKotlin
• Eyeball a GNOME dark session: separator is now a light hairline in the Adwaita flyout

Note: top placement keeps Jewel chrome failed on nucleus-2.5 before this branch — its assertSame on the representation only held while SpellChecker had no warm session. It now probes that the ambient chrome is what ultimately draws, for both placements.