docs: add structure docs

docs/structure.md

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+# PCD Structure
+
+## Operational vs Stateful
+
+PCDs are **operational databases** - they store SQL operations, not data
+snapshots.
+
+| Stateful                       | Operational                                                 |
+| ------------------------------ | ----------------------------------------------------------- |
+| Stores what data should be     | Stores how to create data                                   |
+| `{"name": "HD Quality"}`       | `INSERT INTO quality_profiles (name) VALUES ('HD Quality')` |
+| Merge conflicts when combining | Natural composition through execution order                 |
+| Diffs show state changes       | Diffs show operation changes                                |
+
+## Document Driven SQL
+
+Each entity is stored as a complete SQL document containing all related
+operations.
+
+**Example: A profile document (`profiles/hd-quality.sql`)**
+
+```sql
+-- Profile entity
+INSERT INTO quality_profiles (name, language_id) VALUES (...);
+
+-- Quality list
+INSERT INTO quality_profile_qualities (...);
+
+-- Custom format scores  
+INSERT INTO quality_profile_custom_formats (...);
+
+-- Tags
+INSERT INTO quality_profile_tags (...);
+```
+
+One file = one complete entity with all relationships.
+
+## Directory Structure
+
+```
+database-repo/
+├── pcd.json                           # Manifest
+├── operations/
+│   ├── core/
+│   │   ├── quality_profiles/
+│   │   │   ├── hd-quality.sql
+│   │   │   ├── 4k-remux.sql
+│   │   │   └── efficient-1080p.sql
+│   │   ├── custom_formats/
+│   │   │   ├── dv-hdr10-plus.sql
+│   │   │   ├── dolby-atmos.sql
+│   │   │   └── scene-release.sql
+│   │   └── regular_expressions/
+│   │       ├── dolby-vision-pattern.sql
+│   │       ├── hdr10-plus-pattern.sql
+│   │       └── remux-pattern.sql
+│   └── tweaks/
+│       ├── anime-formats.sql
+│       └── av1-profiles.sql
+└── README.md
+```
+
+Each entity type has its own directory. Each entity gets its own document file.
+
+## Execution Model
+
+PCDs execute as layered operations:
+
+```
+1. Schema (CREATE TABLE statements)
+   ↓
+2. Dependencies (other PCD operations)
+   ↓
+3. Database (this PCD's operations)
+   ↓
+4. User customizations (user's operations)
+   ↓
+Result: Populated database
+```