scxtop: Refactor scheduler rendering into separate module (Phase 4)

Extract all scheduler rendering logic from app.rs into a dedicated
SchedulerRenderer module, continuing the refactoring effort to improve
code organization and maintainability.

This phase extracts 9 methods related to DSQ (Dispatch Queue)
visualization and scheduler statistics rendering.

Changes:
- Extract scheduler rendering methods into render::SchedulerRenderer
- Create stateless static methods with clear public API
- Add comprehensive test suite (8 tests, 100% pass rate)
- Remove old methods from app.rs and delegate to new module
- Fix borrowing issues with theme access in render methods

Modified Files:
  tools/scxtop/src/app.rs
    - Remove dsq_sparkline, dsq_sparklines, dsq_bar, dsq_bars methods
    - Remove render_scheduler_stats, render_scheduler_sparklines,
      render_scheduler_barchart, render_scheduler methods
    - Remove unused resize_sched_events helper
    - Add SchedulerRenderer import and delegation
    - Fix theme borrowing by calling self.theme() directly

  tools/scxtop/src/render/mod.rs
    - Add scheduler module declaration
    - Export SchedulerRenderer

New Files:
  tools/scxtop/src/render/scheduler.rs
    - SchedulerRenderer struct with static methods
    - Public API: render_scheduler_view(), render_scheduler_stats()
    - Private helpers: dsq_sparkline, dsq_sparklines, dsq_bar, dsq_bars
    - Utility methods: gradient5_color, render_error_msg
    - Support for all view states: Sparkline, BarChart, LineGauge
    - Handle all DSQ metrics: lat_us, slice_consumed, vtime, nr_queued

  tools/scxtop/tests/render_scheduler_tests.rs
    - 8 comprehensive tests covering all rendering paths
    - Tests for sparkline/barchart views, edge cases, localization
    - Theme compatibility tests (Default, MidnightGreen, SolarizedDark)
    - Helper function: create_test_dsq_data()

API Design:
  render_scheduler_view() - Main rendering entry point
    - Returns Result<usize> with new max_sched_events value
    - Handles dynamic terminal resizing
    - Supports localization and multiple themes

  render_scheduler_stats() - Render scheduler statistics panel
    - Displays scheduler name, tick rate, dispatch settings
    - Returns Result<()>

Technical Details:
- Fixed type mismatch: dispatch_keep_last/select_cpu_fallback i64→i64
- Fixed borrowing conflict by removing theme variable caching
- Updated all call sites in render() and render_mangoapp()
- Gracefully handle edge cases (missing scheduler, empty DSQ data)
- All methods return Result<T> for proper error handling

Signed-off-by: Daniel Hodges <[email protected]>

Author: hodgesds