Add Nostr Java API reference documentation

docs/reference/nostr-java-api.md

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+# Nostr Java API Reference
+
+This document provides an overview of the public API exposed by the `nostr-java` modules. It lists the major classes, configuration objects and their key method signatures, and shows brief examples of how to use them. Where applicable, links to related [Nostr Improvement Proposals (NIPs)](https://github.com/nostr-protocol/nips) are provided.
+
+## Identity (`nostr-java-id`)
+
+### `Identity`
+Represents a Nostr identity backed by a private key. It can derive a public key and sign `ISignable` objects.
+
+```java
+public static Identity create(PrivateKey privateKey)
+public static Identity create(String privateKey)
+public static Identity generateRandomIdentity()
+public PublicKey getPublicKey()
+public Signature sign(ISignable signable)
+```
+
+**Usage:**
+```java
+Identity id = Identity.generateRandomIdentity();
+PublicKey pub = id.getPublicKey();
+Signature sig = id.sign(event);
+```
+
+## Event Model (`nostr-java-event`)
+
+### Core Types
+- `BaseMessage` – base class for all relay messages.
+- `BaseEvent` – root class for Nostr events.
+- `BaseTag` – helper for tag encoding and decoding.
+
+### Predefined Events
+The `nostr.event` package provides event implementations for many NIPs:
+
+| Class | NIP |
+|-------|-----|
+| `NIP01Event` | [NIP-01](https://github.com/nostr-protocol/nips/blob/master/01.md) – standard text notes. |
+| `NIP04Event` | [NIP-04](https://github.com/nostr-protocol/nips/blob/master/04.md) – encrypted direct messages. |
+| `NIP05Event` | [NIP-05](https://github.com/nostr-protocol/nips/blob/master/05.md) – DNS identifiers. |
+| `NIP09Event` | [NIP-09](https://github.com/nostr-protocol/nips/blob/master/09.md) – event deletion. |
+| `NIP25Event` | [NIP-25](https://github.com/nostr-protocol/nips/blob/master/25.md) – reactions. |
+| `NIP52Event` | [NIP-52](https://github.com/nostr-protocol/nips/blob/master/52.md) – calendar events. |
+| `NIP99Event` | [NIP-99](https://github.com/nostr-protocol/nips/blob/master/99.md) – classified listings. |
+
+### Filters
+`Filters` and related `Filterable` implementations help build subscription requests.
+
+```java
+new Filters(Filterable... filterables)
+List<Filterable> getFilterByType(String type)
+void setLimit(Integer limit)
+```
+
+**Usage:**
+```java
+Filters filters = new Filters(new AuthorFilter(pubKey));
+filters.setLimit(100);
+```
+
+## WebSocket Clients (`nostr-java-client`, `nostr-java-api`)
+
+### `WebSocketClientIF`
+Abstraction over a WebSocket connection to a relay.
+
+```java
+<T extends BaseMessage> List<String> send(T eventMessage) throws IOException
+List<String> send(String json) throws IOException
+void close() throws IOException
+```
+
+### `StandardWebSocketClient`
+Spring `TextWebSocketHandler` based implementation of `WebSocketClientIF`.
+
+```java
+public StandardWebSocketClient(String relayUri)
+public <T extends BaseMessage> List<String> send(T eventMessage) throws IOException
+public List<String> send(String json) throws IOException
+public void close() throws IOException
+```
+
+### `SpringWebSocketClient`
+Wrapper that adds retry logic around a `WebSocketClientIF`.
+
+```java
+public List<String> send(BaseMessage eventMessage) throws IOException
+public List<String> send(String json) throws IOException
+public List<String> recover(IOException ex, String json) throws IOException
+public void close() throws IOException
+```
+
+### `NostrSpringWebSocketClient`
+High level client coordinating multiple relay connections and signing.
+
+```java
+public NostrIF setRelays(Map<String,String> relays)
+public List<String> sendEvent(IEvent event)
+public List<String> sendRequest(List<Filters> filters, String subscriptionId)
+public NostrIF sign(Identity identity, ISignable signable)
+public boolean verify(GenericEvent event)
+public Map<String,String> getRelays()
+public void close()
+```
+
+### Configuration
+- `RetryConfig` – enables Spring Retry support.
+- `RelaysProperties` – maps relay names to URLs via configuration properties.
+- `RelayConfig` – loads `relays.properties` and exposes a `Map<String,String>` bean.
+
+## Encryption and Cryptography
+
+### `MessageCipher`
+Strategy interface for message encryption.
+
+```java
+String encrypt(String message)
+String decrypt(String message)
+```
+
+Implementations:
+- `MessageCipher04` – NIP-04 direct message encryption.
+- `MessageCipher44` – NIP-44 payload encryption.
+
+### `Schnorr`
+Utility for Schnorr signatures (BIP-340).
+
+```java
+static byte[] sign(byte[] msg, byte[] secKey, byte[] auxRand)
+static boolean verify(byte[] msg, byte[] pubKey, byte[] sig)
+static byte[] generatePrivateKey()
+static byte[] genPubKey(byte[] secKey)
+```
+
+### `Bech32`
+Utility for Bech32/Bech32m encoding used by [NIP-19](https://github.com/nostr-protocol/nips/blob/master/19.md).
+
+```java
+static String toBech32(Bech32Prefix hrp, byte[] hexKey)
+static String fromBech32(String str)
+```
+
+## Utilities (`nostr-java-util`)
+
+### `NostrUtil`
+General helper functions.
+
+```java
+static String bytesToHex(byte[] bytes)
+static byte[] hexToBytes(String hex)
+static byte[] sha256(byte[] data)
+static byte[] createRandomByteArray(int len)
+```
+
+### `NostrException`
+Base checked exception for utility methods.
+
+## Examples
+
+### Send a Text Note (NIP-01)
+```java
+Identity id = Identity.generateRandomIdentity();
+NIP01 nip01 = new NIP01(id).createTextNoteEvent("Hello Nostr");
+NostrIF client = NostrSpringWebSocketClient.getInstance(id)
+        .setRelays(Map.of("relay","wss://relay.example"));
+client.sendEvent(nip01.getEvent());
+```
+
+### Encrypted Direct Message (NIP-04)
+```java
+Identity alice = Identity.generateRandomIdentity();
+Identity bob = Identity.generateRandomIdentity();
+NIP04 dm = new NIP04(alice, bob.getPublicKey())
+        .createDirectMessageEvent("secret");
+String plaintext = NIP04.decrypt(bob, dm.getEvent());
+```
+
+### Subscription with Filters
+```java
+Filters filters = new Filters(new AuthorFilter(pubKey));
+NostrIF client = NostrSpringWebSocketClient.getInstance(id);
+List<String> events = client.sendRequest(filters, "sub-id");
+```
+
+---
+This reference is a starting point; consult the source for complete details and additional NIP helpers.