feat: generalize island: true beyond listings

[Mpdreamz]

Summary

Extends island: true from listing-only to any TOC element. Islands render as a ≫ stub in the parent sidebar; clicking swaps the sidebar for a dedicated island nav with a root-first back-link stack.

Stacked on: #3781

cursorful-video-1786623994046.mp4

New placement options for island: true

• Root of docset.yml or any nested toc.yml
• Inline beside - toc: <path> in parent docset.yml/toc.yml (OR-ed with the toc.yml's own flag — can enable, never disable)
• Any - toc: entry in config/navigation.yml (assembler-side override; same OR semantics)
• In isolated builds the docset root with island: true is silently inert (no parent → RendersAsIsland() returns false)
• Nested islands: each island sidebar shows a root-first back-link stack (nav root → each enclosing island → immediate parent)
• Existing listing: … island: true keeps working through the same mechanism

Design

Three old members (IslandListingRoot, IsIslandListing, IslandVisual) on INavigationItem are collapsed into a single bool IsIsland, set via a new IAssignableIslandNavigation interface. The island root of a page is found by walking the Parent chain:

• RendersAsIsland() → item.IsIsland && item.Parent is not null
• FindIslandRoot() → nearest ancestor where RendersAsIsland() is true

The island view model now reuses the same NavigationRenderNode tree the main sidebar uses, plus a back-link stack from NavigationRenderModel.CreateIsland(). The old hand-rolled IslandNavGroup/IslandNavPage types and duplicated writer code are gone. NavigationRenderCache key widened from IRootNavigationItem to INavigationItem — the old per-writer ConcurrentDictionary that collided across repos is deleted.

Tests added

• IslandNavigationTests (7 tests): isolation-level coverage of all goal cases
• IslandSiteNavigationTests (3 tests): assembler-level OR semantics, docset root island after re-parenting, navigation.yml override
• NavigationRenderModelTests: IslandNode_ProjectsAsIslandKind, CreateIsland_BuildsBackLinkStack, CreateIsland_ContentHash tests
• Configuration tests: island: true parsing at all four config levels, private-repo stripping survival

Test plan

• ./build.sh unit-test passes (4000+ tests)
• dotnet format --verify-no-changes clean
• Isolated serve: add island: true to a nested toc.yml, confirm ≫ stub in parent and back-link stack in island sidebar
• Isolated serve: add island: true to docset root, confirm it renders as the main nav (root exemption)
• Assembled build: add island: true to a navigation.yml entry, confirm island behavior with correct path_prefix-based URLs

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