[EPIC] Unified Infrastructure Extensibility: CDK, Aspire, and Custom Provisioning Providers

## Overview

Umbrella spec tying together the infrastructure extensibility work across CDK language support, Aspire polyglot integration, and custom provisioning providers into one cohesive design. The goal is a unified architecture where developers can define infrastructure in any language (C#, TypeScript, Python, Go) and have all azd commands work transparently -- whether through CDK, Aspire AppHost, or custom provisioning extensions.

**Spec status:** In progress. This epic is the cross-cutting design owner — it does NOT execute the work directly. Implementation is tracked in the dependent epics below.

## Related Issues

### Provisioning Provider Extensibility
- #6231 - Provisioning Provider Extensibility (feature request)
- #7465 - Epic: Provisioning Providers in the AZD Extension Framework (implementation epic with 16 sub-issues)

### CDK Language Support
- #7425 - Feature: CDK Language Support -- Author Azure Infrastructure in Any Programming Language (spec + POC PR #7352)

### Aspire Integration
- #7138 - Support Aspire polyglot (non-C#) AppHost projects in azd

### Service Discoverability
- #6229 - Custom Service Discoverability
- #6230 - Resource Discoverability

## Key Design Questions

1. Core provider vs extension for CDK languages
2. Single CDK extension vs per-language extensions
3. How Aspire polyglot detection integrates with the provisioning provider framework
4. azd init UX for selecting infrastructure language
5. Template ecosystem support for multiple IaC languages

## Dependent Epics & Status

| # | Title | Owner | Milestone | Status |
|---|---|---|---|---|
| #7465 | Provisioning Providers in AZD Extension Framework (16 sub-issues) | @wbreza | April 2026 | 🟢 Active |
| #7425 | CDK Language Support | _unowned_ | _none_ | 🔴 Needs owner + milestone |
| #7138 | Aspire polyglot AppHost | @tg-msft @vhvb1989 | On Deck | 🟡 Scheduled |
| #6231 | Provisioning Provider Extensibility (proposal) | _unowned_ | _none_ | 🔴 Proposal — needs decision |
| #6229 | Custom Service Discoverability | _unowned_ | _none_ | 🔴 Proposal |
| #6230 | Resource Discoverability | _unowned_ | _none_ | 🔴 Proposal |

## Owners & Coordination

- @jongio — strategic owner, design alignment
- @tg-msft — Aspire integration
- @wbreza — Extension framework execution (#7465)