docs: Add examples for custom derives and fixed arrays

Added comprehensive example schemas for features implemented in #50 and #51:

- examples/custom_derives.lumos (170+ lines)
  - Demonstrates #[derive(...)] attribute usage
  - Shows Copy, PartialEq, Hash, Default, and other custom derives
  - Context-aware generation (Anchor vs pure Borsh)
  - Covers structs, enums, and mixed scenarios

- examples/fixed_arrays.lumos (210+ lines)
  - Demonstrates [T; N] fixed-size array syntax
  - Common Solana patterns ([u8; 32] for hashes, [u8; 64] for signatures)
  - Nested fixed arrays
  - Size calculation validation

Both examples include detailed comments and best practices.

Related: #50 (custom derives), #51 (fixed arrays)
Phase 5.3 Advanced Type System examples complete.

Author: rz1989s

examples/custom_derives.lumos

@@ -0,0 +1,70 @@
+// Example: Custom Derive Macros in LUMOS
+//
+// This example demonstrates how to use custom derive macros
+// to add additional traits beyond the auto-generated ones.
+
+// Example 1: Basic custom derives for equality comparison
+#[solana]
+#[derive(PartialEq, Eq, Hash)]
+struct PlayerStats {
+    wins: u32,
+    losses: u32,
+    draws: u32,
+}
+
+// Example 2: Custom derives with Anchor account
+// Note: #[account] structs get no auto-derives from Anchor,
+// but you can still add custom derives
+#[solana]
+#[account]
+#[derive(PartialEq, Eq)]
+struct UserAccount {
+    wallet: PublicKey,
+    balance: u64,
+    verified: bool,
+}
+
+// Example 3: Enum with custom derives
+#[solana]
+#[derive(PartialEq, Eq, Hash, Copy)]
+enum GameState {
+    Lobby,
+    InProgress,
+    Paused,
+    Finished,
+}
+
+// Example 4: Complex enum with custom derives
+#[solana]
+#[derive(PartialEq, Eq)]
+enum GameEvent {
+    PlayerJoined(PublicKey),
+    ScoreUpdated {
+        player: PublicKey,
+        new_score: u64,
+    },
+    GameEnded,
+}
+
+// Example 5: Deduplication - if you specify a derive that LUMOS
+// auto-generates (like Debug or Clone), it won't be duplicated
+#[solana]
+#[derive(Debug, Clone, PartialEq)]  // Debug and Clone won't be duplicated
+struct Item {
+    id: u64,
+    name: String,
+}
+
+// Generated Rust code will have:
+// #[derive(BorshSerialize, BorshDeserialize, Debug, Clone, PartialEq, Eq, Hash)]
+// for PlayerStats (auto-generated + custom, deduplicated)
+//
+// #[derive(PartialEq, Eq)]
+// #[account]
+// for UserAccount (only custom derives, Anchor provides serialization)
+//
+// Benefits of custom derives:
+// - PartialEq, Eq: Enable equality comparisons (==, !=)
+// - Hash: Enable use in HashSet and HashMap
+// - Copy: Enable bitwise copying for small types
+// - Ord, PartialOrd: Enable ordering comparisons (<, >, <=, >=)

examples/fixed_arrays.lumos

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+// Fixed-size array examples for LUMOS
+// Issue #51: Add const generics support for fixed-size arrays
+
+#[solana]
+#[account]
+struct FixedArrayExample {
+    // Simple fixed array of primitives
+    data: [u8; 32],
+
+    // Fixed array of Solana types
+    authority_list: [PublicKey; 5],
+
+    // Nested fixed arrays (2D grid)
+    grid: [[u8; 10]; 10],
+
+    // Mixed: fixed array with regular field
+    balance: u64,
+    inventory_slots: [PublicKey; 50],
+}
+
+#[solana]
+struct Transaction {
+    // Common Solana pattern: 32-byte hash
+    hash: [u8; 32],
+
+    // Signature data
+    signature: [u8; 64],
+
+    // Fixed number of signers
+    signers: [PublicKey; 3],
+}
+
+enum GameEvent {
+    // Unit variant
+    Start,
+
+    // Tuple variant with fixed array
+    MultiTransfer([PublicKey; 10], u64),
+
+    // Struct variant with fixed array
+    GridUpdate {
+        position: [u16; 2],
+        cells: [u8; 100],
+    },
+}