feat(ui): port favorite spell bar overflow

Import the retail arrow-only spell bar scrollbar from LayoutDesc, preserve authored end-button extents and HideDisabled behavior in the shared retained widget, and bind each favorite list to its sole horizontal pixel-scroll model. Match retail selection exposure for off-screen spells and pin the behavior with real-fixture conformance tests.

Document the six-slice world-interaction completion program as the active pre-M4 work order.

Co-authored-by: Codex <codex@openai.com>

docs/plans/2026-07-23-world-interaction-completion.md

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+# World interaction completion — pre-M4 program
+
+**Status:** Slice 1 code-complete 2026-07-23; connected visual gate pending.
+**Milestone:** M4 prerequisite/preamble.
+**Architecture:** retained gameplay UI over shared selection, object, and
+interaction state. `GameWindow` remains a composition/callback shell.
+
+## Outcome
+
+Close the remaining retail interaction surfaces before the larger M4
+quest/emote/character-creation bodies begin:
+
+1. Favorite spell bars expose their DAT-authored overflow arrows and scroll
+   through every server-persisted favorite.
+2. The status bar's hand and magnifying-glass controls invoke the same Use and
+   Assess commands as their keyboard paths.
+3. Assessing a creature, player, NPC, or object opens its retail information in
+   the canonical shared main-panel host.
+4. World picking can resolve visible equipped children, such as a character's
+   wielded weapon, while selection markers remain anchored to the picked child.
+5. Vendor use opens the authored vendor surface, publishes its inventory, and
+   supports retail selection/browsing.
+6. Vendor buy/sell transactions, quantities, pending-state ownership, and
+   authoritative inventory reconciliation complete the loop.
+
+The program reuses the existing retained-window host, `SelectionState`,
+`ClientObjectTable`, interaction transaction owner, and server-authoritative
+inventory updates. It does not create parallel panel positions, item tables,
+selection state, or optimistic inventory outcomes.
+
+## Ordered slices
+
+| Slice | Deliverable | Principal owner |
+|---|---|---|
+| 1 | Spell-bar overflow arrows | `SpellcastingUiController` + generic retained scrollbar/list |
+| 2 | Status Use/Assess commands | focused status-bar controller binding to the existing action router |
+| 3 | Assessment information panel | new retained panel controller mounted by the canonical main-panel host |
+| 4 | Equipped-child world picking | pure world-query/picking policy plus presentation anchor |
+| 5 | Vendor browse lifecycle | vendor session/controller plus authored panel |
+| 6 | Vendor transactions | server-authoritative buy/sell command and reconciliation owner |
+
+Each slice begins with named-retail research, produces pseudocode and
+conformance tests, updates the divergence register if required, and lands as a
+separate bisectable commit. A visual gate follows each UI-bearing slice.
+
+Slice 1 now imports both 23-pixel arrow buttons and HideDisabled from the real
+combat fixture, shares the list's single pixel-scroll model, advances one
+32-pixel cell per press, and exposes programmatically selected spells. The
+focused 57-test set, all 3,466 App tests / 3 skips, the Release solution build,
+and all 7,839 solution tests / 5 skips pass. Only the connected visual gate
+remains before Slice 2 starts.
+
+## Slice 1 — spell-bar overflow arrows
+
+### Retail oracle
+
+The authored Magic combat layout contains, inside every favorite-tab group:
+
+- horizontal scrollbar `0x100000B5`, 685×36 at the 800-pixel design width;
+- decrement button `0x10000071`, 23×36;
+- increment button `0x10000072`, 23×36;
+- item list `0x100000B6`, inset by 23 pixels on both sides and 32 pixels high.
+
+The list references the scrollbar through `UIElement_Scrollable` property
+`0x71`. The scrollbar references increment/decrement buttons through
+properties `0x77`/`0x78`, and property `0x79` enables HideDisabled. There is no
+track or thumb in this specific control: the two authored arrows are the whole
+visible scrollbar.
+
+Named retail references and executable pseudocode are recorded in
+[`../research/2026-07-23-retail-spellbar-overflow-pseudocode.md`](../research/2026-07-23-retail-spellbar-overflow-pseudocode.md).
+
+### Implementation plan
+
+1. Generalize `DatWidgetFactory.BuildScrollbar` so horizontal and vertical
+   scrollbars both import the referenced decrement/increment children,
+   including their authored dimensions and HideDisabled property.
+2. Generalize `UiScrollbar` to use distinct authored decrement/increment
+   extents for rendering, hit-testing, track geometry, and dragging. Preserve
+   the existing 16-pixel default for layouts whose button children are absent.
+3. Reproduce retail disabled presentation: a model without overflow rejects
+   pointer input; when HideDisabled is authored, the scrollbar also draws
+   nothing and does not claim hit tests.
+4. In `SpellcastingUiController`, bind each group's scrollbar to its
+   `UiItemList.Scroll`, enable horizontal scrolling, and retain one independent
+   pixel offset per favorite tab.
+5. Match `SpellCastSubMenu::SetSelected`: selecting a spell through keyboard,
+   shortcut, or code scrolls that item into view.
+6. Pin the importer, arrow hit extents, 32-pixel step, no-overflow behavior,
+   controller binding, and selection exposure with focused App tests.
+7. Run the App Release suite, solution Release build, and complete Release
+   suite. Then update the roadmap/memory and request the connected visual gate:
+   place more favorites than fit, scroll both directions, change tabs, and
+   verify arrows disappear on a non-overflowing tab.
+
+### Invariants
+
+- DAT supplies the controls and their artwork; no new spell-bar texture or
+  overlay is invented.
+- The scrollbar and list share one `UiScrollable`; there is no second offset.
+- One arrow press moves one 32-pixel favorite cell, matching
+  `UIElement_ListBox::InqScrollDelta`.
+- Hidden disabled arrows cannot intercept combat-page dragging or clicks.
+- Existing stack, combat-power, chat, inventory, spellbook, and external
+  container scrollbars retain their current behavior.
+- No substantial feature body enters `GameWindow`.