Add capability registry scaffold and seed entries from reuse-surface

Bootstrap registry/indexes/capabilities.yaml and migrate helix_forge
capability entries owned by this repository for federation publishing.

registry/capabilities/capability.feature-control.rollout.md

@@ -0,0 +1,111 @@
+---
+id: capability.feature-control.rollout
+name: Feature Rollout Control
+summary: Gradually expose features to subjects across tenants, domains, groups, or cohorts using rollout rules and staged availability.
+owner: feature-control
+status: draft
+domain: helix_forge
+tags:
+  - feature-control
+  - rollout
+  - planning
+
+maturity:
+  discovery:
+    current: D4
+    target: D6
+    confidence: medium
+    rationale: >
+      Rollout is a distinct bounded behavior from single-point evaluation, with
+      research references in the feature-control domain and maturity standard.
+  availability:
+    current: A2
+    target: A4
+    confidence: low
+    rationale: >
+      Rollout logic may exist in source modules but is not yet consistently
+      exposed as a standalone SDK or API surface distinct from evaluate.
+
+external_evidence:
+  completeness:
+    level: C2
+    name: Partial
+    confidence: low
+    basis: scope_vs_intent_and_consumer_expectations
+    satisfied_expectations:
+      - rollout concepts documented adjacent to feature evaluation
+    broken_expectations:
+      - no dedicated rollout artifact called out separately from evaluate
+      - percentage and cohort rollout variants not indexed independently
+    out_of_scope_expectations:
+      - billing-driven entitlements
+  reliability:
+    level: R1
+    name: Fragile
+    confidence: low
+    basis: consumer_quality_signals
+    known_reliability_risks:
+      - rollout behavior may be conflated with evaluate in consumer mental models
+
+discovery:
+  intent: >
+    Control how features become available over time and across cohorts without
+    conflating rollout policy with authorization or billing.
+  includes:
+    - staged rollout rules
+    - cohort and context targeting for rollout
+    - explainable rollout state
+  excludes:
+    - one-time feature evaluation only
+    - authorization decisions
+    - billing entitlements
+  assumptions:
+    - feature evaluation capability exists for final availability decisions
+  use_cases:
+    - ucc.feature-control.domain-rollout
+  research_memos:
+    - specs/CapabilityMaturityStandard.md
+
+availability:
+  current_level: A2
+  target_level: A4
+  current_artifacts:
+    - feature-control/packages/feature-control-sdk
+  target_artifacts:
+    - feature-control/services/feature-control-api
+  consumption_modes:
+    - source module
+    - SDK
+
+relations:
+  depends_on:
+    - capability.feature-control.evaluate
+  supports: []
+  related_to:
+    - capability.feature-control.visibility
+
+evidence:
+  documentation:
+    - specs/CapabilityMaturityStandard.md
+  tests: []
+  consumer_feedback: []
+  bug_reports: []
+  incidents: []
+
+consumer_guidance:
+  recommended_for:
+    - planning staged feature exposure separate from binary evaluation
+  not_recommended_for:
+    - simple on/off evaluation without rollout semantics
+    - entitlement or billing ownership
+  known_limitations:
+    - distinguish carefully from capability.feature-control.evaluate
+---
+
+# Feature Rollout Control
+
+## Overview
+
+Rollout governs how availability changes over time and across cohorts. It
+complements evaluation, which answers whether a feature is available for a
+subject in a context right now.