Add dev-only CLI drift audit command (stdout-only, no files)

[adrunkhuman]

Problem

Parser drift against live 90minut HTML can go unnoticed between fixture corpus refreshes.
Current tests are deterministic/offline (good), but we do not have a quick developer command to detect upstream structure changes early.

Goal

Add a dev-only drift audit command that checks live pages and reports parser contract drift directly in CLI output.

Scope

• Add a command (e.g. go run ./cmd/driftaudit) that fetches a small fixed sample set:
  • archive page
  • selected league pages
  • selected match pages
• Reuse existing parser/service logic; no new persistence layer.
• Validate key invariants already reflected in tests:
  • seasons extraction + selected season behavior
  • competition extraction order/availability
  • rounds + fixtures extraction
  • timeline side assignment sanity
• Print report to stdout only (no files created).
• Exit codes:
  • 0: no drift detected
  • 1: drift/contract failures detected
  • 2: fetch/runtime error
• Optional --json mode that prints JSON to stdout.

Non-goals

• No automatic updates to fixtures/tests.
• No CI requirement for live network checks.
• No disk caching/persistence.

Acceptance criteria

• Running drift audit gives a concise CLI summary with pass/fail counts and failing checks.
• Drift conditions return non-zero exit code (1) without mutating repository files.
• Runtime/network failures return exit code 2.
• Existing offline parser tests remain unchanged and deterministic.