Implement symbol names in backtrace functionality

[Copilot]

This PR implements the TODO to print symbol names in the backtrace functionality located in kernel/src/utils/backtrace.rs.

Changes Made

1. Created Symbol Management Module (kernel/src/symbols.rs)

• Symbol table storage: Uses BTreeMap for efficient symbol lookups by address
• ELF symbol parsing: Attempts to load symbols from kernel ELF file via limine KernelFileRequest
• Robust symbol resolution: Handles exact matches and offset-based lookups within symbol boundaries
• Fallback mechanism: Provides basic placeholder symbols if ELF parsing fails

2. Enhanced Backtrace Output (kernel/src/utils/backtrace.rs)

• Symbol resolution: Calls symbols::get_symbol(rip) for each stack frame
• Improved formatting: Shows frame numbers and function names with offsets
• Safety features: Limits to 32 frames to prevent infinite loops
• Better UX: Much more informative than raw addresses alone

3. Integration and Testing

• Boot initialization: Symbols module initialized during kernel startup
• Comprehensive testing: Added test functions to verify symbol resolution
• Error handling: Graceful fallbacks when symbols aren't available

Before vs After

Before (TODO):

let symbol = ""; // crate::symbols::get_symbol(rip);
dbg!("0x{:x} {}", rip, symbol);

After (Implemented):

let symbol = crate::symbols::get_symbol(rip).unwrap_or_else(|| "<unknown>".to_string());
dbg!("#{}: 0x{:x} {}", frame_count, rip, symbol);

Example backtrace output:

==================== BACKTRACE ====================
#0: 0x100500 kernel_main+0x500
#1: 0x101100 panic_handler+0x100  
#2: 0x102080 log_backtrace+0x80
==================================================

Technical Details

The implementation uses limine's KernelFileRequest to access the kernel ELF file at runtime, then parses the symbol table to extract function names and addresses. Symbol lookup uses efficient binary search through the sorted address space, with support for both exact matches and reasonable offset-based matches.

The symbol resolution algorithm:

1. Checks for exact address matches within symbol boundaries
2. Falls back to nearest symbol with reasonable offset (< symbol size + 1KB)
3. Returns <unknown> for addresses that can't be resolved

Fixes #10.

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