add readme to src/xrpld/app/wasm (#6340)

* add readme to src/xrpl/app/wasm

* important block

* respond to copilot

src/xrpld/app/wasm/README.md

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+# WASM Module for Programmable Escrows
+
+This module provides WebAssembly (WASM) execution capabilities for programmable
+escrows on the XRP Ledger. When an escrow is finished, the WASM code runs to
+determine whether the escrow conditions are met, enabling custom programmable
+logic for escrow release conditions.
+
+For the full specification, see
+[XLS-0102: WASM VM](https://xls.xrpl.org/xls/XLS-0102-wasm-vm.html).
+
+## Architecture
+
+The module follows a layered architecture:
+
+```
+┌─────────────────────────────────────────────────────────────┐
+│                 WasmEngine (WasmVM.h)                       │
+│         runEscrowWasm(), preflightEscrowWasm()              │
+│              Host function registration                     │
+├─────────────────────────────────────────────────────────────┤
+│                 WasmiEngine (WasmiVM.h)                     │
+│            Low-level wasmi interpreter integration          │
+├─────────────────────────────────────────────────────────────┤
+│    HostFuncWrapper          │       HostFuncImpl            │
+│   C-style WASM bridges      │   C++ implementations         │
+├─────────────────────────────────────────────────────────────┤
+│                   HostFunc (Interface)                      │
+│            Abstract base class for host functions           │
+└─────────────────────────────────────────────────────────────┘
+```
+
+### Key Components
+
+- **`WasmVM.h` / `detail/WasmVM.cpp`** - High-level facade providing:
+  - `WasmEngine` singleton that wraps the underlying WASM interpreter
+  - `runEscrowWasm()` - Execute WASM code for escrow finish
+  - `preflightEscrowWasm()` - Validate WASM code during preflight
+  - `createWasmImport()` - Register all host functions
+
+- **`WasmiVM.h` / `detail/WasmiVM.cpp`** - Low-level integration with the
+  [wasmi](https://github.com/wasmi-labs/wasmi) WebAssembly interpreter:
+  - `WasmiEngine` - Manages WASM modules, instances, and execution
+  - Memory management and gas metering
+  - Function invocation and result handling
+
+- **`HostFunc.h`** - Abstract `HostFunctions` base class defining the interface
+  for all callable host functions. Each method returns
+  `Expected<T, HostFunctionError>`.
+
+- **`HostFuncImpl.h` / `detail/HostFuncImpl*.cpp`** - Concrete
+  `WasmHostFunctionsImpl` class that implements host functions with access to
+  `ApplyContext` for ledger state queries. Implementation split across files:
+  - `HostFuncImpl.cpp` - Core utilities (updateData, checkSignature, etc.)
+  - `HostFuncImplFloat.cpp` - Float/number arithmetic operations
+  - `HostFuncImplGetter.cpp` - Field access (transaction, ledger objects)
+  - `HostFuncImplKeylet.cpp` - Keylet construction functions
+  - `HostFuncImplLedgerHeader.cpp` - Ledger header info access
+  - `HostFuncImplNFT.cpp` - NFT-related queries
+  - `HostFuncImplTrace.cpp` - Debugging/tracing functions
+
+- **`HostFuncWrapper.h` / `detail/HostFuncWrapper.cpp`** - C-style wrapper
+  functions that bridge WASM calls to C++ `HostFunctions` methods. Each host
+  function has:
+  - A `_proto` type alias defining the function signature
+  - A `_wrap` function that extracts parameters and calls the implementation
+
+- **`ParamsHelper.h`** - Utilities for WASM parameter handling:
+  - `WASM_IMPORT_FUNC` / `WASM_IMPORT_FUNC2` macros for registration
+  - `wasmParams()` helper for building parameter vectors
+  - Type conversion between WASM and C++ types
+
+## Host Functions
+
+Host functions allow WASM code to interact with the XRP Ledger. They are
+organized into categories:
+
+- **Ledger Information** - Access ledger sequence, timestamps, hashes, fees
+- **Transaction & Ledger Object Access** - Read fields from the transaction
+  and ledger objects (including the current escrow object)
+- **Keylet Construction** - Build keylets to look up various ledger object types
+- **Cryptography** - Signature verification and hashing
+- **Float Arithmetic** - Mathematical operations for amount calculations
+- **NFT Operations** - Query NFT properties
+- **Tracing/Debugging** - Log messages for debugging
+
+For the complete list of available host functions, their WASM names, and gas
+costs, see the [XLS-0102 specification](https://xls.xrpl.org/xls/XLS-0102-wasm-vm.html)
+or `detail/WasmVM.cpp` where they are registered via `WASM_IMPORT_FUNC2` macros.
+For method signatures, see `HostFunc.h`.
+
+## Gas Model
+
+Each host function has an associated gas cost. The gas cost is specified when
+registering the function in `detail/WasmVM.cpp`:
+
+```cpp
+WASM_IMPORT_FUNC2(i, getLedgerSqn, "get_ledger_sqn", hfs, 60);
+//                                                        ^^ gas cost
+```
+
+WASM execution is metered, and if the gas limit is exceeded, execution fails.
+
+## Entry Point
+
+The WASM module must export a function with the name defined by
+`ESCROW_FUNCTION_NAME` (currently `"finish"`). This function:
+
+- Takes no parameters (or parameters passed via host function calls)
+- Returns an `int32_t`:
+  - `1` (or positive): Escrow conditions are met, allow finish
+  - `0` (or negative): Escrow conditions are not met, reject finish
+
+## Adding a New Host Function
+
+To add a new host function, follow these steps:
+
+### 1. Add to HostFunc.h (Base Class)
+
+Add a virtual method declaration with a default implementation that returns an
+error:
+
+```cpp
+virtual Expected<ReturnType, HostFunctionError>
+myNewFunction(ParamType1 param1, ParamType2 param2)
+{
+    return Unexpected(HostFunctionError::INTERNAL);
+}
+```
+
+### 2. Add to HostFuncImpl.h (Declaration)
+
+Add the method override declaration in `WasmHostFunctionsImpl`:
+
+```cpp
+Expected<ReturnType, HostFunctionError>
+myNewFunction(ParamType1 param1, ParamType2 param2) override;
+```
+
+### 3. Implement in detail/HostFuncImpl\*.cpp
+
+Add the implementation in the appropriate file:
+
+```cpp
+Expected<ReturnType, HostFunctionError>
+WasmHostFunctionsImpl::myNewFunction(ParamType1 param1, ParamType2 param2)
+{
+    // Implementation using ctx (ApplyContext) for ledger access
+    return result;
+}
+```
+
+### 4. Add Wrapper to HostFuncWrapper.h
+
+Add the prototype and wrapper declaration:
+
+```cpp
+using myNewFunction_proto = int32_t(uint8_t const*, int32_t, ...);
+wasm_trap_t*
+myNewFunction_wrap(void* env, wasm_val_vec_t const* params, wasm_val_vec_t* results);
+```
+
+### 5. Implement Wrapper in detail/HostFuncWrapper.cpp
+
+Implement the C-style wrapper that bridges WASM to C++:
+
+```cpp
+wasm_trap_t*
+myNewFunction_wrap(void* env, wasm_val_vec_t const* params, wasm_val_vec_t* results)
+{
+    // Extract parameters from params
+    // Call hfs->myNewFunction(...)
+    // Set results and return
+}
+```
+
+### 6. Register in WasmVM.cpp
+
+Add the function registration in `setCommonHostFunctions()` or
+`createWasmImport()`:
+
+```cpp
+WASM_IMPORT_FUNC2(i, myNewFunction, "my_new_function", hfs, 100);
+//                                   ^^ WASM name       ^^ gas cost
+```
+
+> [!IMPORTANT]
+> New host functions MUST be amendment-gated in `WasmVM.cpp`.
+> Wrap the registration in an amendment check to ensure the function is only
+> available after the corresponding amendment is enabled on the network.