chore: rename fixture families by purpose

[rlippmann]

What changed

• Renamed fixture family directories from version-based names to purpose-based names:
  • tests/fixtures/v2 -> tests/fixtures/conformance
  • tests/fixtures/v3 -> tests/fixtures/engine-regression
• Kept fixture subtree layout unchanged.
• Updated only required references in tests/docs/agent guidance to new paths.
• Preserved fixture JSON contents (rename-only for fixture data files).

Why this was needed

• Makes fixture families easier to understand and maintain by describing purpose instead of historical version labels, without changing fixture behavior.