Enforce the performance-budget contract with measured gates

[tph-kds]

Source: skills/deckforge/references/performance-budget.md defines the budget contract.

Gap: the budget is advisory; the reference app CI does not measure and gate on it.

Acceptance:

• Lighthouse (or equivalent) measurement of LCP, bundle size, and interaction latency against the contract thresholds.
• Regression gate: measured values outside the budget fail CI.
• Budget values derived from the performance-budget contract (single source of truth), not hardcoded in CI.