docs: add documentation for EVM opcode offset constants

[DeVikingMark]

What I did

Added documentation comments to EVM opcode offset constants (PUSH_OFFSET, DUP_OFFSET, SWAP_OFFSET) in the assembler core module to improve code readability and developer understanding.

How I did it

Added explanatory comments above each constant definition explaining:

• Their purpose as base values for EVM opcode calculation
• The formula used: OFFSET + instruction_number
• Specific examples showing how opcodes are derived (PUSH0 = 0x5F, DUP1 = 0x80, SWAP1 = 0x90)

How to verify it

1. Check that the constants are still accessible: python3 -c "from vyper.evm.assembler.core import PUSH_OFFSET, DUP_OFFSET, SWAP_OFFSET; print(f'{PUSH_OFFSET}, {DUP_OFFSET}, {SWAP_OFFSET}')"
2. Verify Python syntax: python3 -m py_compile vyper/evm/assembler/core.py
3. Confirm the comments explain the EVM opcode encoding logic clearly

Commit message

docs: add documentation for EVM opcode offset constants

Description for the changelog

Added documentation comments to EVM assembler constants to clarify opcode encoding logic.

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