refactor: use NonZeroU16 for StepId/StringId, colorize dump output

[zharinov]

Summary

• Use NonZeroU16 for StepId and StringId to make terminal state (0) unrepresentable
• Add .get() accessor and ::new() constructor for type safety
• Remove StepId::ACCEPT and Label::ACCEPT sentinels - terminal is now absence (empty successors)
• Colorize dump output using the merged Colors module
• Use shared formatting utilities from the merged format module

Changes

• bytecode/ids.rs: StepId and StringId now wrap NonZeroU16
• bytecode/dump.rs: Colorized output, uses format module utilities
• bytecode/module.rs: Rename decode_step_view → decode_step, add get_by_index for strings
• compile/expressions.rs, emit/: Updated to use new API

Test plan

• All 681 tests pass
• Clippy passes