chore: prepare 0.6.1 release

[banteg]

Summary

• promote the current Unreleased notes to 0.6.1 dated 2026-07-25
• bump package and lock metadata from 0.6.0 to 0.6.1
• update the schema-v4 producer identity integration assertion for the release version

Release contents

• source validation now uses the nearest declared project environment without import guessing or source retries
• report schema v4 records producer, compiler authority and identity, dependency context, exact attempts, and typed failure origins
• explicit --source-vyper selection and arguments remain authoritative inside declared project environments

Validation

• uv lock --check
• uv run --locked ruff check src tests scripts/corpus.py scripts/release_notes.py scripts/release_preflight.py
• uv run --locked pytest — 863 passed
• scripts/smoke-wheel.sh — built, installed, and exercised vyupgrade 0.6.1
• uv run --locked python scripts/release_notes.py v0.6.1
• uv run --locked python scripts/release_preflight.py v0.6.1 --dist dist