Pixel-based scroll-position seam (IScrollOffsetSource) — step 1 of #33

[andrewtheart]

Step 1 of the editor scroll/render series (#33)

This is the foundational piece agreed in #33: a small pixel-based scroll-position seam.

What & why

Today the editor's vertical scroll position is stored in legacy scrollbar units
(SingleLineHeight / DefaultVerticalScrollSensitivity), while horizontal is in pixels. That
mismatch makes the later work (word-wrap-aware scrolling, long-line horizontal virtualization, and
especially the eventual swap to a real ScrollViewer for diagonal 2-axis touchpad scrolling) into a
whole-consumer unit rewrite.

This PR introduces IScrollOffsetSource — a single funnel for the scroll position, expressed in
pixels — and routes ScrollManager's offset reads/writes through it. Phase 1 backs it with the
existing two ScrollBar primitives via a ScrollBarOffsetSource adapter that does the
pixel↔scrollbar-unit conversion internally. The pure arithmetic lives in ScrollOffsetMath so it's
unit-testable in isolation (a ScrollBar can't be created headless — this is exactly where a
"scroll is 4× too fast" regression would hide).

Behavior

No user-visible behavior change. VerticalScroll / HorizontalScroll keep their existing public
semantics; every mutator produces the same scrollbar values as before. Extent/viewport setup
(VerticalScrollbar_Loaded, EnsureHorizontalScrollBounds) is intentionally left writing the
scrollbars directly — the adapter reads those same scrollbars — to keep this PR a pure offset-seam
refactor. The Extent/Viewport/ChangeView members on the interface are the forward-looking
surface that the ScrollViewer backend (a later PR) will drop into.

Changes

• new Core/ScrollOffsetMath.cs — pure pixel↔scrollbar-unit conversions.
• new Core/IScrollOffsetSource.cs — the seam interface + ScrollBarOffsetSource adapter.
• Core/ScrollManager.cs — routes offset access through OffsetSource.
• tests ScrollOffsetMathTests.cs — round-trips, sensitivity, extent, and clamping.

Verification

TextControlBox (library) builds clean; the test project builds clean (x64). Happy to adjust naming
or scope to your taste before the next PR in the series.