feat(lithoglyph/gql-dt): implement FFI stubs — wire db_open, db_close, parse, execute

Replace all TODO/placeholder stub functions in the GQL-DT Zig FFI layer
with working implementations that allocate and manage opaque handle state
(DbState, QueryState, SchemaState, ResultState) on the heap.

Zig FFI (ffi/zig/src/main.zig):
- gqldt_db_open: validates path, allocates DbState, returns opaque handle
- gqldt_db_close: frees DbState backing memory
- gqldt_parse / gqldt_parse_inferred: store query text in QueryState
- gqldt_typecheck: marks query as type-checked (precondition for execute)
- gqldt_execute: verifies typecheck precondition, allocates ResultState
- gqldt_serialize_cbor: encodes query as CBOR text string (RFC 8949)
- gqldt_serialize_json: produces {"query":"..."} wrapper
- gqldt_deserialize_cbor: decodes CBOR text string back to QueryState
- gqldt_get_schema: allocates SchemaState for collection
- gqldt_validate_permissions: validates user_id, delegates to Idris2
- gqldt_query_free / gqldt_schema_free / gqldt_result_free: proper cleanup
- gqldt_version: returns "0.1.0"
- Slot allocation helpers (alloc_slot/read_slot/free_slot/bits64_to_ptr)
  for Idris2 pointer marshalling

Idris2 ABI (src/GQLdt/ABI/Foreign.idr):
- Replace all ?impl_pending holes (dbOpen, dbClose, parse, parseInferred,
  typecheck, execute, dbOpenSafe, parseSafe) with implementations using
  allocSlot/readSlot/freeSlot + Data.So.choose pattern (zero believe_me)
- Add validateDbPath and validateQueryStr inline validators
- Add handle pointer extractors (extractDbPtr, extractQueryPtr, extractSchemaPtr)

Build (build.zig):
- Migrate to Zig 0.15.2 API (addLibrary/createModule)
- Build both static and shared library artifacts

Tests:
- Replace template placeholders in integration_test.zig with real GQL-DT
  function signatures
- All 21 unit tests pass, both libraries compile clean

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

lithoglyph/gql-dt/ffi/zig/build.zig

@@ -1,27 +1,51 @@
 // SPDX-License-Identifier: PMPL-1.0-or-later
-// SPDX-FileCopyrightText: 2025 Jonathan D.A. Jewell (@hyperpolymath)
+// SPDX-FileCopyrightText: 2025-2026 Jonathan D.A. Jewell (@hyperpolymath)
 //
-// build.zig - GQL-DT FFI Build Configuration
+// build.zig - GQL-DT FFI Build Configuration (Zig 0.15.2+)
+//
+// Builds the libgqldt shared library (C ABI) and unit tests.
 
 const std = @import("std");
 
 pub fn build(b: *std.Build) void {
     const target = b.standardTargetOptions(.{});
     const optimize = b.standardOptimizeOption(.{});
 
-    //Build tests
-    const tests = b.addTest(.{
-        .name = "gqldt-tests",
-        .target = target,
-        .optimize = optimize,
+    // Static library (libgqldt.a)
+    const static_lib = b.addLibrary(.{
+        .name = "gqldt",
+        .root_module = b.createModule(.{
+            .root_source_file = b.path("src/main.zig"),
+            .target = target,
+            .optimize = optimize,
+        }),
+        .linkage = .static,
     });
+    b.installArtifact(static_lib);
 
-    tests.root_module.addAnonymousImport("main", .{
-        .root_source_file = b.path("src/main.zig"),
+    // Shared library (libgqldt.so / libgqldt.dylib)
+    const shared_lib = b.addLibrary(.{
+        .name = "gqldt",
+        .root_module = b.createModule(.{
+            .root_source_file = b.path("src/main.zig"),
+            .target = target,
+            .optimize = optimize,
+        }),
+        .linkage = .dynamic,
+    });
+    b.installArtifact(shared_lib);
+
+    // Unit tests (from main.zig internal tests)
+    const unit_tests = b.addTest(.{
+        .name = "gqldt-tests",
+        .root_module = b.createModule(.{
+            .root_source_file = b.path("src/main.zig"),
+            .target = target,
+            .optimize = optimize,
+        }),
     });
-    tests.linkLibC();
 
-    const run_tests = b.addRunArtifact(tests);
+    const run_unit_tests = b.addRunArtifact(unit_tests);
     const test_step = b.step("test", "Run unit tests");
-    test_step.dependOn(&run_tests.step);
+    test_step.dependOn(&run_unit_tests.step);
 }