docs: Add Light Protocol compression integration guide

Author: SwenSchaeferjohann

docs/LIGHT_COMPRESSION.md

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+# Light Protocol Compression Integration
+
+## Fork Changes
+
+This fork adds compressed token support to anchor-spl's `token_interface`:
+
+```rust
+// anchor-spl/src/token_interface.rs
+pub const COMPRESSED_TOKEN_ID: Pubkey = ...;
+static IDS: [Pubkey; 3] = [spl_token::ID, spl_token_2022::ID, COMPRESSED_TOKEN_ID];
+```
+
+## TS SDK
+
+The TS SDK adds `decompressIfNeeded()` to method builders:
+
+```typescript
+await program.methods
+  .swap(amount)
+  .decompressIfNeeded()
+  .rpc();
+```
+
+## Rust Macros (light-sdk-macros)
+
+### `#[add_compressible_instructions]`
+
+Generates compress/decompress instructions for listed accounts.
+
+```rust
+#[add_compressible_instructions(
+    PoolState = (POOL_SEED, ctx.accounts.amm_config, ctx.accounts.token_0_mint, ctx.accounts.token_1_mint),
+)]
+#[program]
+pub mod my_program { ... }
+```
+
+### `#[derive(LightFinalize)]` + `#[compressible]`
+
+Auto-compresses PDAs at instruction end.
+
+```rust
+#[derive(Accounts, LightFinalize)]
+#[instruction(params: MyParams)]
+pub struct MyInstruction<'info> {
+    #[account(mut)]
+    pub creator: Signer<'info>,
+
+    #[account(init, payer = creator, space = 8 + MyAccount::INIT_SPACE, seeds = [...], bump)]
+    #[compressible(
+        address_tree_info = params.address_tree_info,
+        output_tree = params.output_state_tree_index
+    )]
+    pub my_account: Account<'info, MyAccount>,
+
+    pub compression_config: AccountInfo<'info>,
+}
+```
+
+### `#[light_instruction(params)]`
+
+Auto-calls `light_finalize()` at instruction end.
+
+```rust
+#[light_instruction(params)]
+pub fn create_account(ctx: Context<MyInstruction>, params: MyParams) -> Result<()> {
+    // your logic
+    Ok(())  // light_finalize auto-called here
+}
+```
+
+### `#[light_mint]` (single mint)
+
+Creates a compressed mint at instruction end.
+
+```rust
+#[derive(Accounts, LightFinalize)]
+#[instruction(params: MyParams)]
+pub struct CreateMint<'info> {
+    #[account(mut)]
+    pub creator: Signer<'info>,
+
+    /// CHECK: Mint signer PDA that seeds the mint address
+    pub mint_signer: UncheckedAccount<'info>,
+
+    /// CHECK: The mint account to be created
+    #[account(mut)]
+    #[light_mint(
+        mint_signer = mint_signer,
+        authority = authority,
+        decimals = 9,
+        address_tree_info = params.mint_address_tree_info,
+        output_tree = params.output_state_tree_index
+    )]
+    pub my_mint: UncheckedAccount<'info>,
+
+    pub authority: UncheckedAccount<'info>,
+    pub compression_config: AccountInfo<'info>,
+}
+```
+
+## Required Program Setup
+
+```rust
+use light_sdk_types::CpiSigner;
+use light_macros::derive_light_cpi_signer;
+
+pub const LIGHT_CPI_SIGNER: CpiSigner = derive_light_cpi_signer!("YOUR_PROGRAM_ID");
+```
+
+## Params Struct Requirements
+
+Your params struct (first arg of `#[instruction(...)]`) must have:
+
+```rust
+pub struct MyParams {
+    pub address_tree_info: PackedAddressTreeInfo,  // for each compressible/mint
+    pub output_state_tree_index: u8,
+    pub proof: ValidityProof,  // required for mint creation
+}
+```
+
+## Current Limitations
+
+- **Multiple mints**: Not supported via macro. Use `light_ctoken_sdk::ctoken::CreateCMintCpi` directly with CPI context batching.
+- **Mixed PDAs + mints**: Not supported via macro. Handle mints manually.
+- **Token accounts**: Use `light_ctoken_sdk` directly (`CreateCTokenAccountCpi`, `CTokenMintToCpi`).
+
+## Gotchas
+
+1. **Fee payer field**: Must be named `fee_payer`, `payer`, or `creator`
+2. **compression_config field**: Required as `AccountInfo<'info>`
+3. **remaining_accounts**: Light system accounts come via remaining_accounts - client must pass them in v2 format
+4. **PackedAddressTreeInfo fields**: Uses `address_merkle_tree_pubkey_index`, `address_queue_pubkey_index`, `root_index`
+
+## raydium-cp-swap Usage
+
+Currently uses:
+- `#[compressible]` on `pool_state` for auto-compression
+- Manual `CreateCTokenAccountCpi` for token vaults
+- Manual mint handling (LP mint created by client, no `#[light_mint]`)