feat: add bytecode IDs, constants, and header

[zharinov]

Summary

• Add foundational bytecode types for the binary format
• constants.rs: magic bytes (PTKQ), version, section alignment constants
• ids.rs: StepId, StringId, QTypeId newtypes with safe indexing
• header.rs: 64-byte file header struct with section offsets

Context

First PR in a stack implementing bytecode loading (split from larger change for reviewability):

feat/bytecode-ids-header      ← this PR
  └── feat/bytecode-sections
        └── feat/bytecode-effects
              └── feat/bytecode-instructions
                    └── feat/bytecode-module

Test plan

• cargo check passes
• Existing tests unaffected