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

internal/acl/README.md

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+# ACL (Access Control List)
+
+Access control at the TCP connection level with IP/CIDR, timezone, and country-based filtering.
+
+## Overview
+
+The ACL package provides network-level access control by wrapping TCP listeners and validating incoming connections against configurable allow/deny rules. It integrates with MaxMind GeoIP for geographic-based filtering and supports access logging with notification batching.
+
+### Primary consumers
+
+- `internal/entrypoint` - Wraps the main TCP listener for connection filtering
+- Operators - Configure rules via YAML configuration
+
+### Non-goals
+
+- HTTP request-level filtering (handled by middleware)
+- Authentication or authorization (see `internal/auth`)
+- VPN or tunnel integration
+
+### Stability
+
+Stable internal package. The public API is the `Config` struct and its methods.
+
+## Public API
+
+### Exported types
+
+```go
+type Config struct {
+    Default    string                     // "allow" or "deny" (default: "allow")
+    AllowLocal *bool                      // Allow private/loopback IPs (default: true)
+    Allow      Matchers                   // Allow rules
+    Deny       Matchers                   // Deny rules
+    Log        *accesslog.ACLLoggerConfig // Access logging configuration
+
+    Notify struct {
+        To             []string      // Notification providers
+        Interval       time.Duration // Notification frequency (default: 1m)
+        IncludeAllowed *bool         // Include allowed in notifications (default: false)
+    }
+}
+```
+
+```go
+type Matcher struct {
+    match MatcherFunc
+}
+```
+
+```go
+type Matchers []Matcher
+```
+
+### Exported functions and methods
+
+```go
+func (c *Config) Validate() gperr.Error
+```
+
+Validates configuration and sets defaults. Must be called before `Start`.
+
+```go
+func (c *Config) Start(parent task.Parent) gperr.Error
+```
+
+Initializes the ACL, starts the logger and notification goroutines.
+
+```go
+func (c *Config) IPAllowed(ip net.IP) bool
+```
+
+Returns true if the IP is allowed based on configured rules. Performs caching and GeoIP lookup if needed.
+
+```go
+func (c *Config) WrapTCP(lis net.Listener) net.Listener
+```
+
+Wraps a `net.Listener` to filter connections by IP.
+
+```go
+func (matcher *Matcher) Parse(s string) error
+```
+
+Parses a matcher string in the format `{type}:{value}`. Supported types: `ip`, `cidr`, `tz`, `country`.
+
+## Architecture
+
+### Core components
+
+```mermaid
+graph TD
+    A[TCP Listener] --> B[TCPListener Wrapper]
+    B --> C{IP Allowed?}
+    C -->|Yes| D[Accept Connection]
+    C -->|No| E[Close Connection]
+
+    F[Config] --> G[Validate]
+    G --> H[Start]
+    H --> I[Matcher Evaluation]
+    I --> C
+
+    J[MaxMind] -.-> K[IP Lookup]
+    K -.-> I
+
+    L[Access Logger] -.-> M[Log & Notify]
+    M -.-> B
+```
+
+### Connection filtering flow
+
+```mermaid
+sequenceDiagram
+    participant Client
+    participant TCPListener
+    participant Config
+    participant MaxMind
+    participant Logger
+
+    Client->>TCPListener: Connection Request
+    TCPListener->>Config: IPAllowed(clientIP)
+
+    alt Loopback IP
+        Config-->>TCPListener: true
+    else Private IP (allow_local)
+        Config-->>TCPListener: true
+    else Cached Result
+        Config-->>TCPListener: Cached Result
+    else Evaluate Allow Rules
+        Config->>Config: Check Allow list
+        alt Matches
+            Config->>Config: Cache true
+            Config-->>TCPListener: Allowed
+        else Evaluate Deny Rules
+            Config->>Config: Check Deny list
+            alt Matches
+                Config->>Config: Cache false
+                Config-->>TCPListener: Denied
+            else Default Action
+                Config->>MaxMind: Lookup GeoIP
+                MaxMind-->>Config: IPInfo
+                Config->>Config: Apply default rule
+                Config->>Config: Cache result
+                Config-->>TCPListener: Result
+            end
+        end
+    end
+
+    alt Logging enabled
+        Config->>Logger: Log access attempt
+    end
+```
+
+### Matcher types
+
+| Type     | Format            | Example               |
+| -------- | ----------------- | --------------------- |
+| IP       | `ip:address`      | `ip:192.168.1.1`      |
+| CIDR     | `cidr:network`    | `cidr:192.168.0.0/16` |
+| TimeZone | `tz:timezone`     | `tz:Asia/Shanghai`    |
+| Country  | `country:ISOCode` | `country:GB`          |
+
+## Configuration Surface
+
+### Config sources
+
+Configuration is loaded from `config/config.yml` under the `acl` key.
+
+### Schema
+
+```yaml
+acl:
+  default: "allow"              # "allow" or "deny"
+  allow_local: true             # Allow private/loopback IPs
+  log:
+    log_allowed: false          # Log allowed connections
+  notify:
+    to: ["gotify"]              # Notification providers
+    interval: "1m"              # Notification interval
+    include_allowed: false      # Include allowed in notifications
+```
+
+### Hot-reloading
+
+Configuration requires restart. The ACL does not support dynamic rule updates.
+
+## Dependency and Integration Map
+
+### Internal dependencies
+
+- `internal/maxmind` - IP geolocation lookup
+- `internal/logging/accesslog` - Access logging
+- `internal/notif` - Notifications
+- `internal/task/task.go` - Lifetime management
+
+### Integration points
+
+```go
+// Entrypoint uses ACL to wrap the TCP listener
+aclListener := config.ACL.WrapTCP(listener)
+http.Server.Serve(aclListener, entrypoint)
+```
+
+## Observability
+
+### Logs
+
+- `ACL started` - Configuration summary on start
+- `log_notify_loop` - Access attempts (allowed/denied)
+
+Log levels: `Info` for startup, `Debug` for client closure.
+
+### Metrics
+
+No metrics are currently exposed.
+
+## Security Considerations
+
+- Loopback and private IPs are always allowed unless explicitly denied
+- Cache TTL is 1 minute to limit memory usage
+- Notification channel has a buffer of 100 to prevent blocking
+- Failed connections are immediately closed without response
+
+## Failure Modes and Recovery
+
+| Failure                           | Behavior                              | Recovery                                      |
+| --------------------------------- | ------------------------------------- | --------------------------------------------- |
+| Invalid matcher syntax            | Validation fails on startup           | Fix configuration syntax                      |
+| MaxMind database unavailable      | GeoIP lookups return unknown location | Default action applies; cache hit still works |
+| Notification provider unavailable | Notification dropped                  | Error logged, continues operation             |
+| Cache full                        | No eviction, uses Go map              | No action needed                              |
+
+## Usage Examples
+
+### Basic configuration
+
+```go
+aclConfig := &acl.Config{
+    Default:    "allow",
+    AllowLocal: ptr(true),
+    Allow: acl.Matchers{
+        {match: matchIP(net.ParseIP("192.168.1.0/24"))},
+    },
+    Deny: acl.Matchers{
+        {match: matchISOCode("CN")},
+    },
+}
+if err := aclConfig.Validate(); err != nil {
+    log.Fatal(err)
+}
+if err := aclConfig.Start(parent); err != nil {
+    log.Fatal(err)
+}
+```
+
+### Wrapping a TCP listener
+
+```go
+listener, err := net.Listen("tcp", ":443")
+if err != nil {
+    log.Fatal(err)
+}
+
+// Wrap with ACL
+aclListener := aclConfig.WrapTCP(listener)
+
+// Use with HTTP server
+server := &http.Server{}
+server.Serve(aclListener)
+```
+
+### Creating custom matchers
+
+```go
+matcher := &acl.Matcher{}
+err := matcher.Parse("country:US")
+if err != nil {
+    log.Fatal(err)
+}
+
+// Use the matcher
+allowed := matcher.match(ipInfo)
+```