docs: use pinned Foundry toolchain for semver-lock

[ayaanoncrypto]

Summary

• Replace the unpinned foundryup recovery instruction with the repository-pinned mise toolchain.
• • Document the .mise.toml version pin as the source of truth for semver-lock and snapshot reproducibility.
• • Keep a fallback path for contributors who do not use mise, with an explicit version check.
    Closes docs: foundryup recommendation can install a version different from the repository-pinned Foundry toolchain #403

Validation

• git diff --check

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/2 Emergency override progress: 0/5 These approvals do not satisfy normal or CODEOWNER requirements because the commit identity is unverified.	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 1 Sum 2