feat: declarative view engine + 4 new views (PRISM milestone)

[flyingrobots]

Summary

Refactor hardcoded views into a declarative view engine and add 4 new high-value views.

Deliverables

Declarative view engine

• declareView(name, config) — compile prefix/type filter configs into views
• Refactor existing roadmap, architecture, backlog views to declarative configs

New views

• milestone — progress tracking: tasks per milestone, completion %, blockers
• traceability — spec-to-implementation gap analysis: which specs lack implementations?
• blockers — transitive blocking chain resolution with cycle detection
• onboarding — topologically-sorted reading order for new engineers

Tests

• Declarative config compilation
• All 4 new views with graph fixtures

Context

PRISM milestone — depends on BEDROCK (complete). Part of TECH-PLAN §9.3.