chore: migrate fixtures to conformance and harden source selection

[rlippmann]

What changed

• Renamed TS fixture family path from tests/fixtures/v2 to tests/fixtures/conformance.
• Updated fixture loader, sync/check scripts, and docs/test labels to use conformance.
• Made fixture source explicit via FIXTURES_SOURCE with no implicit fallback.
• Updated CI to compare against a pinned Python source-of-truth commit and explicit fixture path.
• Aligned TS replacement clarify behavior with Python conformance fixture expectations.
• Added and consolidated AGENTS guidance so conformance fixture updates are sync-driven and not hand-edited.

Why

• Keep TS fixture workflow deterministic and aligned with Python as source of truth.
• Ensure conformance fixture updates are mechanical and reproducible.