Add measurement contracts to Marketing OS

[douglas951]

What changed

• Added marketing-science/measurement, a reusable contract for strategies, campaigns, launches, experiments, and ongoing programs.
• Added a measurement doctrine to the foundation layer.
• Required the contract in launches/gtm-plan and the flagship launch canon.
• Updated the dependency architecture and repository coverage.

Why

The OS named useful outcome classes but did not force baselines, metric formulas, targets, guardrails, instrumentation, attribution limits, or decision rules. This change makes measurement operational while preserving the distinction between Marketing demand creation and Growth conversion.

Impact

Every measured initiative now starts with a decision, uses one primary metric per objective, records data limitations, and pre-commits the action for success, mixed evidence, failure, or inconclusive data. Retros end in a decision instead of a dashboard recap.

Validation

• git diff --check
• Skill structure and dependency-path checks
• every-style mechanical pass
• ai-check editorial pass