feat(post1-T-4.1): MCP protocol_version 2 — natural JSON value encoding (0.7.x)

[escapeboy]

Summary

Breaking change for the 0.7.x branch — bumps TOOL_RESPONSE_PROTOCOL_VERSION from 1 to 2.

• Option::None → null (was {"option": "None"})
• Option::Some(v) → v unwrapped (was {"option": "Some", "value": v})
• Result::Ok(v) → {"ok": v} (was {"result": "Ok", "value": v})
• Result::Err(v) → {"err": v} (was {"result": "Err", "value": v})
• Records → named-field objects {"field1": v1, "field2": v2} using the module type table (was positional {"type":"record","type_id":N,"fields":[...]})
• Enum variants → {"VariantName": payload} for tagged, "VariantName" for unit (was {"type":"enum","type_id":N,"variant":"Name","payload":...})

Fallback for missing type info: records fall back to positional arrays; enums fall back to the v1 shape.

Why 0.7.x only

This is a breaking wire-format change that cannot go on the 1.x LTS master branch. 0.7.x is the designated channel for LTS-incompatible improvements.

Files

• crates/boruna-mcp/src/tools/run.rs — new format_value_v2(value, module), updated format_value_capped to accept &Module
• crates/boruna-mcp/src/tools/mod.rs — version constant 1 → 2, lock test updated
• docs/reference/mcp-server.md — updated Stability section + value encoding table
• CHANGELOG.md — unreleased entry

Test plan

• cargo test --workspace passes (all crates, locally verified)
• cargo clippy --workspace -- -D warnings clean
• cargo fmt --all -- --check clean

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