feat(pcbnew): collab adds footprints (s-expr blob) + vias/zones (native)

Footprint add reconstructs from the bare `(footprint …)` s-expr clipboard blob via
CLIPBOARD_IO (SetWriter/SetReader redirect it to a string for headless/wasm). The
non-footprint `(kicad_pcb …)` envelope parse (parseBOARD) is asyncify-fragile in wasm
— commit.Add of an envelope-parsed item silently stages nothing and subsequent virtual
dispatch traps ("index out of bounds"), the same wall that deferred the eeschema symbol
blob — so vias and zones reconstruct NATIVELY instead: itemToJson emits drill/layer-pair
for a PCB_VIA and the outline polygon for a ZONE, and makeItem builds a fresh PCB_VIA /
ZONE. flushDiff attaches the blob only to types that need it (footprints/board graphics);
tracks/vias/zones skip it. Includes the dangling-parent fix for the envelope path
(SetParent before delete) which footprints don't hit.

Build: kicad_clipboard.h pulls in the generated pcb_lexer.h (emitted into the common
build subdir by make_lexer) — added -I${KICAD_BUILD}/common to the embind include path.

Tests: sample board gains a via + zone; snapshot asserts their native fields are emitted,
and three round-trip add tests (footprint via blob, via/zone native) delete then re-add
each by uuid and confirm it returns at the same position. 6 passed, 1 skipped, 0 aborts.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/kicad/build-kicad-target.sh

@@ -395,6 +395,9 @@ if [ -f "${EMBIND_SRC}" ]; then
     log_info "Compiling Embind bindings (${APP_NAME})..."
     # Use the same includes and flags that KiCad uses
     KICAD_INCLUDES="-I${KICAD_BUILD} -I${KICAD_DIR}/include -I${KICAD_DIR}/${KICAD_SUBDIR} -I${KICAD_DIR}/common"
+    # Generated DSN-lexer headers (e.g. pcb_lexer.h, used transitively via kicad_clipboard.h →
+    # pcb_io_kicad_sexpr_parser.h) are emitted into the common build subdir by make_lexer.
+    KICAD_INCLUDES+=" -I${KICAD_BUILD}/common"
     KICAD_INCLUDES+=" -I${KICAD_DIR}/libs/core/include -I${KICAD_DIR}/libs/kimath/include -I${KICAD_DIR}/libs/kiplatform/include"
     KICAD_INCLUDES+=" -I${KICAD_DIR}/thirdparty/clipper2/Clipper2Lib/include"
     KICAD_INCLUDES+=" -I${KICAD_DIR}/thirdparty/nlohmann_json"