perf: compose 30% faster — move semantics, BFS offsets, zero-alloc hex formatting

- compose.cpp: emitLine takes LineMeta&& (move, not copy) at all 22 call sites
- compose.cpp: reserve meta/text buffers, BFS offset computation O(N) vs O(N*D)
- compose.cpp: pre-compute typeNameLens[], merge global width loops
- format.cpp: bytesToHex uses stack buffer + lookup table (zero heap allocs)
- format.cpp: hexVal single QString::asprintf instead of 2-string concat
- editor.cpp: guard hover updates during applyDocument (stale index safety)
- core.h: assertion on makeArrayElemSelId negative index
- format.cpp: assertion on extractBits overflow
- main.cpp: tree lines enabled by default
- bench_large_class: add 2000-field benchComposeLarge test

Benchmark: 500 fields 0.70→0.51ms (27%), 2000 fields 2.28→1.57ms (31%)

src/core.h

@@ -575,6 +575,7 @@ static constexpr uint64_t kArrayElemMask  = 0x3FFF000000000000ULL;  // 14 bits
 
 // Encode an array element selection ID: nodeId | kArrayElemBit | (elemIdx << 48)
 inline uint64_t makeArrayElemSelId(uint64_t nodeId, int elemIdx) {
+    Q_ASSERT(elemIdx >= 0);
     return nodeId | kArrayElemBit | ((uint64_t)(elemIdx & 0x3FFF) << kArrayElemShift);
 }
 inline int arrayElemIdxFromSelId(uint64_t selId) {