Respect physical display width when scrolling

[BanjoR]

Automatic issue closing

Fixes #427

Summary

• Bound display writes and refresh pacing to the physical display width.
• Capped physical width at MAX_CHARACTER_COLS so wide display drivers cannot overrun the row buffer.
• Prevented narrow OLED rows from wrapping into subsequent rows.

References

This is the independently testable display regression slice for CommandStation issue #427. It is separate from PR #512, PR #511, PR #414, and issue #383. No superseded BanjoR PR was identified.

Scope and exclusions

• Only Display.cpp is changed in the validated source branch.
• Personal integration code and unrelated protocol behavior are explicitly out of scope.

Validation evidence

• Clean task-owned checkout from DCC-EX/CommandStation-EX:master.
• Host/static regression checks and git diff --check passed.
• PASS - Exact default python -m platformio run in a clean task checkout with an isolated PlatformIO core completed for all five configured environments: mega2560, ESP32, Nucleo-F411RE, Nucleo-F446RE, and Nucleo-F429ZI.
• Exact-head fork CI: PASS - CI run 31950680704 completed successfully for head 8ade964.
• No repository hardware or display runtime was available.

Hardware validation

Hardware validation: Not run—no hardware available.

Maintainer bench criteria: on each supported display width, exercise scrolling at the narrowest OLED width, the normal character width, and a width above MAX_CHARACTER_COLS; verify no row-buffer overwrite, no wrap into the next row, and unchanged refresh behavior on supported display hardware.

This PR is ready for maintainer review; hardware/display validation remains outstanding; local five-environment compilation is complete.