docs: add per package README for implementation details (AI generated with human review)

internal/route/rules/presets/README.md

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+# Rule Presets
+
+Provides embedded, pre-configured rule sets for common routing patterns.
+
+## Overview
+
+The `internal/route/rules/presets` package provides embedded YAML rule configurations that can be reused across routes. Presets are compiled into the binary and loaded at runtime via `sync.Once` initialization.
+
+### Primary Consumers
+
+- **Route configuration**: Applies preset rules to routes
+- **WebUI**: Provides default rules for web applications
+- **API gateway**: Common patterns for API routes
+
+### Non-goals
+
+- Does not modify presets at runtime
+- Does not provide dynamic preset loading
+- Does not support preset inheritance/overriding
+
+### Stability
+
+Internal package. Preset content is stable but may change between versions.
+
+## Public API
+
+### Exported Functions
+
+```go
+// GetRulePreset retrieves a preset by name
+func GetRulePreset(name string) (rules.Rules, bool)
+```
+
+**Contract:**
+
+- Uses `sync.Once` for one-time initialization
+- Returns a copy of the preset rules
+- Second return value indicates if preset exists
+
+## Architecture
+
+### Core Components
+
+```mermaid
+classDiagram
+    class GetRulePreset {
+        <<function>>
+        +sync.Once
+        +rulePresets map
+    }
+
+    class RulePreset {
+        <<embedded>>
+        +fs.FS
+    }
+
+    class webui {
+        <<preset>>
+        +Login page rule
+        +Protected routes rule
+        +API proxy rule
+        +Auth proxy rule
+    }
+
+    GetRulePreset --> RulePreset : loads
+    RulePreset --> webui : contains
+```
+
+### Preset Loading Flow
+
+```mermaid
+sequenceDiagram
+    participant App as Application
+    participant Once as sync.Once
+    participant FS as Embedded FS
+    participant Parser as YAML Parser
+    participant Map as rulePresets
+
+    App->>Once: GetRulePreset("webui")
+    Once->>Once: once.Do(initPresets)
+    Once->>FS: ReadDir("presets/")
+    FS-->>Once: List files
+    loop Each file
+        Once->>FS: ReadFile(filename)
+        FS-->>Once: YAML content
+        Once->>Parser: Parse YAML
+        Parser-->>Once: rules.Rules
+        Once->>Map: Store in map
+    end
+    Map-->>Once: Loaded presets
+    Once-->>App: Rules copy
+```
+
+## Preset Files
+
+### Directory Structure
+
+```
+internal/route/rules/presets/
+├── embed.go          //go:embed *.yml
+├── webui.yml         // WebUI preset
+└── README.md
+```
+
+### webui Preset
+
+The default preset for GoDoxy WebUI:
+
+**What it does:**
+
+1. Allows unauthenticated access to `/login`
+1. Requires auth for most paths, excluding static assets and auth endpoints
+1. Proxies `/api/v1/*` to backend
+1. Rewrites and proxies `/auth/*` to backend
+
+## Dependency and Integration Map
+
+| Dependency               | Purpose                         |
+| ------------------------ | ------------------------------- |
+| `internal/route/rules`   | Rules engine for preset content |
+| `internal/serialization` | YAML parsing                    |
+| `sync`                   | One-time initialization         |
+
+## Observability
+
+### Logs
+
+- DEBUG: Preset loading errors
+- WARN: Missing preset files
+
+## Security Considerations
+
+- Presets are compiled into binary (immutable at runtime)
+- Environment variable substitution (`${VAR}`) supports secure configuration
+- No runtime preset modification possible
+
+## Failure Modes and Recovery
+
+| Failure             | Behavior             | Recovery              |
+| ------------------- | -------------------- | --------------------- |
+| Preset file missing | Returns (nil, false) | Check preset exists   |
+| YAML parse error    | Panic during init    | Fix preset YAML       |
+| Unknown preset name | Returns (nil, false) | Use valid preset name |
+
+## Usage Examples
+
+### Loading a Preset
+
+```go
+import "github.com/yusing/godoxy/internal/route/rules/presets"
+
+rules, ok := presets.GetRulePreset("webui")
+if !ok {
+    return fmt.Errorf("preset not found")
+}
+
+// Apply rules to a handler
+handler := rules.BuildHandler(upstreamHandler)
+```
+
+### Creating a Custom Preset
+
+```yaml
+# internal/route/rules/presets/api-gateway.yml
+- name: cors headers
+  on: method OPTIONS
+  do: |
+    set header Access-Control-Allow-Origin *
+    set header Access-Control-Allow-Methods "GET, POST, PUT, DELETE, OPTIONS"
+    error 204 "No Content"
+
+- name: auth required
+  on: !path /health
+  do: require_auth
+```
+
+Then load it:
+
+```go
+rules, ok := presets.GetRulePreset("api-gateway")
+```
+
+### Using Preset in Route Config
+
+```yaml
+routes:
+  webapp:
+    target: http://localhost:3000
+    rules: |
+      - name: include webui preset
+        do: include webui
+      - name: additional rule
+        on: path /custom/*
+        do: proxy http://custom:8080
+```
+
+## Limitations
+
+- Presets are read-only after initialization
+- No runtime preset modification
+- All presets loaded at first access (no lazy loading)
+- No preset merging (caller must handle)