feat(graphics): Phase 3 - 2D graphics primitives + xtask QEMU isolation (#98)

* feat(graphics): add 2D graphics primitives module (Phase 3)

Implements Phase 3 of the graphics stack with 2D drawing primitives:

Core types:
- Color struct with RGB support and BGR/RGB pixel format conversion
- Point and Rect structs for geometry
- Canvas trait for abstracted drawing surface

Drawing functions:
- draw_hline/draw_vline: Optimized horizontal/vertical lines
- draw_line: General line drawing using Bresenham's algorithm
- draw_rect: Rectangle outline
- fill_rect: Filled rectangle with row optimization
- draw_circle/fill_circle: Circle using midpoint algorithm

Integration:
- ShellFrameBuffer implements Canvas trait
- draw_test_pattern() demo function for testing

All drawing functions handle bounds checking and clipping automatically.
Coordinates use i32 to allow negative values (clipped at canvas boundary).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* fix(xtask): use per-worktree PID tracking instead of global pkill

Replace all instances of `pkill -9 qemu-system-x86_64` with worktree-specific
PID file tracking. This allows multiple worktrees to run boot-stages tests
simultaneously without killing each other's QEMU processes.

Changes:
- Add get_qemu_pid_file() to generate unique PID file path per worktree
- Add signal_worktree_qemu() to send signals to specific QEMU process
- Add kill_worktree_qemu() and term_worktree_qemu() helpers
- Add save_qemu_pid() to track QEMU PID on startup
- Replace all 6 pkill calls with worktree-specific PID-based killing

PID files are stored at /tmp/breenix-qemu-<worktree-hash>.pid using
a hash of the current working directory for uniqueness.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: ryanbreen

kernel/src/graphics/mod.rs

@@ -3,5 +3,6 @@
 //! Provides framebuffer abstractions used by the kernel graphics stack.
 
 pub mod double_buffer;
+pub mod primitives;
 
 pub use double_buffer::DoubleBufferedFrameBuffer;