feat: Phase E.3 — WASM target (Emscripten embind + drag-and-drop demo)

Compile the core Parquet reader/writer to WebAssembly via Emscripten
embind, enabling browser-side Parquet inspection with zero server
dependencies. Includes AES-256 PME encrypted file support.

Platform guards:
- forge.hpp: exclude mmap_reader.hpp and streaming_sink.hpp on __EMSCRIPTEN__
- wal.hpp: exclude wal_mapped_segment.hpp on __EMSCRIPTEN__

CMake:
- SIGNET_BUILD_WASM option + signet_wasm target with embind link flags
- wasm configure/build preset (commercial enabled, no external codecs)

New files:
- wasm/signet_wasm.cpp: embind bindings for SchemaBuilder, Schema,
  WriterOptions, ParquetWriter (6 typed write methods), ParquetReader
  (6 typed read methods + readColumnAsStrings), MEMFS helpers, version(),
  openEncrypted() with footer key + column key + AAD prefix support
- wasm/demo.html: self-contained drag-and-drop Parquet preview with
  paginated data table (50 rows/page, 10K max loaded), schema inspector,
  file metadata, encrypted file toggle (footer key, column key, AAD prefix,
  per-column keys), dark theme

Output: signet_wasm.js (104K) + signet_wasm.wasm (521K)
Native regression: 394/394 tests pass (guards are __EMSCRIPTEN__-only)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Johnson-Ogundeji

CMakeLists.txt

@@ -44,6 +44,7 @@ set(SIGNET_EVAL_WARN_PCT_2 "90" CACHE STRING
     "Default evaluation-tier second warning threshold percentage")
 set(SIGNET_COMMERCIAL_LICENSE_HASH "" CACHE STRING
     "64-bit FNV-1a hash literal for SIGNET_COMMERCIAL_LICENSE_KEY (example: 0x0123abcd)")
+option(SIGNET_BUILD_WASM     "Build WASM target via Emscripten embind" OFF)
 option(SIGNET_MINIMAL_DEPS    "Disable ALL optional external dependencies (for embedded/microcontroller builds)" OFF)
 
 if(SIGNET_BUILD_AI_AUDIT)
@@ -474,6 +475,36 @@ if(SIGNET_BUILD_FFI)
     message(STATUS "SIGNET_BUILD_FFI=ON: C FFI static library enabled")
 endif()
 
+# ---------------------------------------------------------------------------
+# WASM — Emscripten embind target (browser-side Parquet preview)
+# ---------------------------------------------------------------------------
+if(SIGNET_BUILD_WASM AND EMSCRIPTEN)
+    add_executable(signet_wasm wasm/signet_wasm.cpp)
+    target_link_libraries(signet_wasm PRIVATE signet::forge)
+    target_compile_features(signet_wasm PRIVATE cxx_std_20)
+    target_link_options(signet_wasm PRIVATE
+        --bind
+        -sWASM=1
+        -sALLOW_MEMORY_GROWTH=1
+        -sMODULARIZE=1
+        -sEXPORT_NAME=SignetForge
+        -sENVIRONMENT=web
+        -sFORCE_FILESYSTEM=1
+        "-sEXPORTED_RUNTIME_METHODS=['FS']"
+        -fexceptions
+    )
+    target_compile_options(signet_wasm PRIVATE -fexceptions)
+    set_target_properties(signet_wasm PROPERTIES
+        SUFFIX ".js"
+    )
+    install(FILES
+        ${CMAKE_CURRENT_BINARY_DIR}/signet_wasm.js
+        ${CMAKE_CURRENT_BINARY_DIR}/signet_wasm.wasm
+        DESTINATION share/signet_forge/wasm
+    )
+    message(STATUS "SIGNET_BUILD_WASM=ON: Emscripten WASM target enabled")
+endif()
+
 # ---------------------------------------------------------------------------
 # Fuzz harnesses (libFuzzer — requires Clang)
 # ---------------------------------------------------------------------------

CMakePresets.json

@@ -179,6 +179,25 @@
                 "CMAKE_CXX_FLAGS": "-fprofile-instr-generate -fcoverage-mapping",
                 "CMAKE_EXE_LINKER_FLAGS": "-fprofile-instr-generate"
             }
+        },
+        {
+            "name": "wasm",
+            "displayName": "WASM (Emscripten embind, browser-side Parquet + PME encryption)",
+            "generator": "Ninja",
+            "binaryDir": "${sourceDir}/build-wasm",
+            "cacheVariables": {
+                "CMAKE_BUILD_TYPE": "Release",
+                "SIGNET_BUILD_WASM": "ON",
+                "SIGNET_ENABLE_COMMERCIAL": "ON",
+                "SIGNET_ENABLE_ZSTD": "OFF",
+                "SIGNET_ENABLE_LZ4": "OFF",
+                "SIGNET_ENABLE_GZIP": "OFF",
+                "SIGNET_ENABLE_PQ": "OFF",
+                "SIGNET_BUILD_TESTS": "OFF",
+                "SIGNET_BUILD_BENCHMARKS": "OFF",
+                "SIGNET_BUILD_EXAMPLES": "OFF",
+                "SIGNET_BUILD_TOOLS": "OFF"
+            }
         }
     ],
     "buildPresets": [
@@ -222,7 +241,8 @@
         { "name": "server",     "configurePreset": "server" },
         { "name": "server-pq",  "configurePreset": "server-pq" },
         { "name": "fuzz",       "configurePreset": "fuzz" },
-        { "name": "coverage",   "configurePreset": "coverage" }
+        { "name": "coverage",   "configurePreset": "coverage" },
+        { "name": "wasm",       "configurePreset": "wasm" }
     ],
     "testPresets": [
         {

include/signet/ai/wal.hpp

@@ -1170,6 +1170,9 @@ class WalManager {
 // Included here so detail_mmap:: helpers have access to the signet::forge
 // namespace already opened above; WalMmapWriter produces files readable by
 // WalReader without any changes to the reader.
+// Excluded on Emscripten — WalMmapWriter requires std::thread + POSIX mmap.
 // ---------------------------------------------------------------------------
+#ifndef __EMSCRIPTEN__
 #include "signet/ai/wal_mapped_segment.hpp"
+#endif
 

include/signet/forge.hpp

@@ -22,7 +22,9 @@
 /// @note Commercial AI audit and compliance modules (audit_chain, decision_log,
 ///       inference_log, MiFID II/EU AI Act reporters) are conditionally included
 ///       only when `SIGNET_ENABLE_COMMERCIAL` is defined and non-zero.
-/// @note The mmap_reader module is excluded on Windows (`_WIN32`).
+/// @note The mmap_reader module is excluded on Windows (`_WIN32`) and Emscripten (`__EMSCRIPTEN__`).
+/// @note The streaming_sink and wal_mapped_segment modules are excluded on Emscripten
+///       (they require `std::thread`, unavailable in single-threaded WASM).
 
 /// @name Core: error handling, types, schema, statistics, memory, Thrift, column I/O
 /// @{
@@ -81,7 +83,7 @@
 /// @name Page Index + Memory-Mapped I/O
 /// @{
 #include "signet/column_index.hpp"
-#ifndef _WIN32
+#if !defined(_WIN32) && !defined(__EMSCRIPTEN__)
 #include "signet/mmap_reader.hpp"
 #endif
 /// @}
@@ -96,7 +98,9 @@
 /// @name Streaming WAL + Async Compaction (Apache 2.0)
 /// @{
 #include "signet/ai/wal.hpp"
+#ifndef __EMSCRIPTEN__
 #include "signet/ai/streaming_sink.hpp"
+#endif
 /// @}
 
 /// @name Feature Store: point-in-time feature retrieval (Apache 2.0)