ecode: add customizable debugger panel layouts

- add a reusable right-side panel managed at the application level
  - migrate debugger tabs to UITabWidgetSplitter instances
  - support moving and splitting debugger tabs across bottom and right panels
  - restrict debugger tab drops to debugger-owned panel targets
  - serialize debugger tab layouts, panel state, and table column widths
  - add pixel and percentage column-width modes to UIAbstractTableView
  - support XML/CSS configuration and an optional width-mode context menu
  - preserve percentage totals across resizing, fit-to-content, minimum widths,
    scrollbar changes, and pixel-density rounding
  - add splitter edge hiding while preserving splitter-always-show precedence
  - use EventConnection ownership for safe widget and controller lifetimes
  - validate serialized splitter layouts with structural limits
  - display project-relative source paths in debugger stack frames
  - add ecode localization strings, CSS documentation, tests, and agent rules

.agent/rules/project-introduction.md

@@ -17,9 +17,24 @@ When working on this project, rely on the following resources to understand exis
 *   **Implementation Examples:** A wide variety of examples showing how to use the library are located in `src/examples/`.
 *   **General Context:** The `README.md` at the root directory contains deeper project details.
 
+## Localization
+
+The locale catalogs under `bin/assets/i18n/` belong to **ecode**; the other applications are not
+currently localized. Whenever an ecode feature introduces or changes a user-facing i18n key, add or
+update that key in every catalog in this directory. Do not rely only on the fallback string embedded
+in the source. Keep all locale files structurally valid and verify that every supported catalog
+contains the new or renamed key.
+
 ## C++ Virtual Method Style
 Follow the convention already used by the class being edited. In particular, when a class declares
 virtual methods without the `override` specifier, do not introduce `override` on new methods in that
 class. Mixing the styles can enable Clang's inconsistent-missing-override warnings for the existing
 declarations. A class-wide conversion is a separate change and must update all applicable methods
 together.
+
+## Namespace Style
+
+Follow eepp's established namespace style: prefer the appropriate `using namespace EE::...`
+declarations and unqualified eepp type names, such as `UISplitter`, over repeatedly spelling fully
+qualified names such as `EE::UI::UISplitter`. Keep explicit qualification only where it is required
+to resolve ambiguity or avoid importing an unusually broad namespace into an unsuitable scope.