feat: improve error messages with more context

[EmperorOrokuSaki]

Issue

Several error messages throughout the codebase lack sufficient context to help users understand what went wrong and how to fix it.

Current State

Examples of generic error messages:

• "no valid entry block found" - doesn't explain why
• InvalidJumpTarget(usize) - only shows PC without context
• Generic parsing errors without position information

Proposed Solution

Enhance error messages with:

• More descriptive explanations
• Context about what was expected vs what was found
• Suggestions for resolution where applicable
• Position information for parsing errors

Examples:

"No valid entry block found: instruction sequence may be empty or contains only terminal instructions"
"Invalid jump target 0x{:x}: target must point to a JUMPDEST instruction within bounds [0x0, 0x{:x}]"

Benefits

• Better user experience
• Easier debugging
• More helpful error reporting
• Reduced support overhead
• Self-documenting error conditions

Files to Modify

• crates/core/src/cfg_ir.rs
• crates/transforms/src/util.rs
• crates/core/src/decoder.rs

Priority

Medium - User experience improvement