Document memory expansion tracking for MSTORE/MSTORE8

[Copilot]

The issue calls for a clear plan to implement correct EVM memory expansion tracking, especially around MSTORE/MSTORE8 so runtime bounds checks can rely on memory_len. This PR adds a focused write-up that ties EVM semantics to Jet’s current runtime design.

• Write-up: memory expansion semantics
  • Summarizes EVM memory growth rules, word rounding, and MSIZE byte tracking.
• Write-up: Jet integration plan
  • Details required updates for MSTORE/MSTORE8 and other memory-touching opcodes.
  • Outlines safe expansion, overflow handling, and memory_len updates.
• Write-up: verification guidance
  • Lists targeted test cases for word-aligned expansion.

new_size = ceil((offset + size) / 32) * 32

Original prompt

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This section details on the original issue you should resolve

<issue_title>Memory length tracking from Mstore and Mstore8.</issue_title>
<issue_description>

jet/crates/jet_runtime/src/builtins.rs

Line 164 in f844206

// TODO: Validate ret_offset + ret_len <= memory_len once memory_len is tracked by MSTORE

// TODO: Validate ret_offset + ret_len <= memory_len once memory_len is tracked by MSTORE

I think it's time that we tackle this issue. Let's first understand exactly what needs to be done to make this work correctly and safely, make a write-up in /docs/tasks explaining everything that will need to be done.</issue_description>

<agent_instructions>Review the documentation that we have about the EVM and opcodes, specifically looking at information about how memory expansion works, and information about the opcodes MSTORE and MSTORE8. Create a writeup explaining how to resolve this issue, how to properly track memory length and memory expansion in a way that's semantically correct with what the EVM does. docs/ext/evm/evm.md and docs/ext/evm/opcodes.json</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Memory length tracking from Mstore and Mstore8. #32

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Review changes with  

[Copilot]

Pull request overview

Adds a task write-up describing how Jet should correctly track EVM memory expansion (in bytes, word-rounded) so memory_len becomes reliable for runtime bounds checks and future opcode work (especially around MSTORE/MSTORE8).

Changes:

• Documents EVM memory expansion sizing rules (word rounding, MSIZE byte tracking, and cost formula reference).
• Describes Jet’s current memory_ptr/memory_len/memory_cap model and the gap (stores don’t update memory_len).
• Proposes a shared expansion helper + integration points across memory-touching opcodes, with suggested test cases.