fix: align Linux setup and oracle vocabulary

[abrichr]

What changed

• Run Playwright installation and dependency checks with the exact Python interpreter that runs Flow.
• Quote the interpreter path in generated Linux remedies.
• Present the canonical Seal Oracle ladder at the tutorial and documentation boundary.
• Describe automatic effect-contract proposals as a MockMed fixture feature. Real deployments still require review and an independent verifier.
• Build a current, self-bound test copy of the finite Flow receipt instead of letting the hosted-gate tests expire on a fixed date.
• Update the locked pip version to 26.2, which clears GHSA-qwm4-qh6w-59xr.

The persisted Flow v1 VerificationTier values do not change. Production qualification admission code also does not change. The committed signed receipt fixture remains immutable.

Verification

• Browser setup: 19 passed, 4 skipped.
• Tutorial and Oracle checks: 16 passed.
• Hosted release gate and receipt checks: 45 passed.
• The exact lock check passes.
• The loopback tutorial test passes outside the filesystem sandbox.
• Ruff check and format check pass.