docs(spec): Tab strip position (top/bottom/left/right) — closes #835

[sashankh]

This is a spec-only PR per @DHowett's guidance — no code changes, just the design document at doc/specs/#835 - Tab Strip Position.md.

Closes #835 (also resolves duplicates #9082, #9100, #10939, #18837 once implementation lands).

Summary

The spec documents the proposed theme.window.tabPosition setting with four values: top, bottom, left, right — adopting the design from @zadjii-msft's FHL prototype branch (dev/migrie/fhl-spring-2026/side-tabs). All four positions get equal treatment in the spec:

• Top (default) — bit-for-bit unchanged from today; uses the existing titlebar-integrated tab strip
• Bottom — row-reorder of the existing layout; tab strip below the content
• Left / Right — three-column layout (tab strip / draggable splitter / content) using a custom XAML re-template (VerticalTabViewStyle.xaml; the "Vertical" prefix is scope-accurate — it applies only to Left/Right, not to Top/Bottom)

The spec covers:

• The new enum on WindowTheme and why it lives there rather than GlobalAppSettings or per-profile
• XAML re-template strategy using MUX's documented Style/ControlTemplate extensibility point (only for Left/Right modes)
• Why imperative grid manipulation in TerminalPage is required today: MUX 2.8 has no TabStripPlacement on TabView (see microsoft/microsoft-ui-xaml#300, open since 2018), and WT keeps _tabContent as a sibling of TabView at the page root rather than inside TabView.TabContentPresenter
• All four position layouts described mathematically (verified on a working prototype: LeftMargin=421, RightMargin=420, BottomMargin=76, TopMargin=81 px)
• 4 honest "Potential Issues" including the runtime-toggle architectural limitation (_ApplyTabPosition() is monotonic) and the right-side accent bar nit
• 7 "Future considerations": collapsible sidebar, clockwise rotation hotkey, hover-to-expand, RTL FlowDirection, telemetry, status icons, acrylic/Mica

Prototype status

A working prototype that implements this spec exists locally (not part of this PR). It has been validated end-to-end on a clean dev box:

• Build clean, 143/143 SettingsModel unit tests pass (incl. the new ParseTabPosition round-trip)
• 21/21 UIA-based e2e tests pass (idempotent test suite, separate file)
• All 4 positions render correctly on cold-restart
• The original "launch crash" from @zadjii-msft's machine was confirmed environmental (bisected to a v1 commit that previously launched fine), not a code defect

Happy to iterate on the spec text, then file separate PRs for the implementation once the design is approved.

PR Checklist

• Closes Feature request: Enable customization for tabs on bottom/right/left #835
• Tests added/passed (N/A — spec-only)
• Documentation updated (this PR IS the documentation)
• Schema updated (separate PR will follow — schema additions for tabPosition are isolated as a +11-line follow-up commit)

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