M5.6a: memory report (library.memory, ui.inspect memory) and opt-in DHAT heap profile

Per-cache bytes: decoded thumbnail/preview/full sources, rendered previews,
GPU device buffers (allocated/pooled/retired), loupe stage caches (CPU and
GPU) and textures. lightcraft-cli --features dhat-heap adds live/peak heap and
writes dhat-heap.json.

Baseline (14 raws, loupe, 12 steps): peak RSS 1.9-2.3 GB, footprint 2.5-2.6 GB;
heap peak 634 MB after import (probes decode every raw), 930 MB during grid
thumbnails (parallel decodes); GPU free pool grows to 2 GB (exact-size reuse).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/query.rs

@@ -135,6 +135,9 @@ pub fn specs() -> Vec<CommandSpec> {
             }
             Ok(json!({"enabled": lightcraft_gpu::enabled(), "available": lightcraft_gpu::available(), "adapter": lightcraft_gpu::adapter_name()}))
         }),
+        cmd!(query "library.memory", "Memory Usage", [], None, "{} — bytes held by each cache (decoded sources, rendered previews, GPU buffers; heap when instrumented)", always, |s, _| {
+            Ok(serde_json::to_value(s.memory_report()).unwrap_or_default())
+        }),
         cmd!(query "journal.list", "Command Journal", [], None, "{limit?}", always, |s, p| {
             let lim = p.get("limit").and_then(Value::as_u64).unwrap_or(100) as usize;
             let start = s.journal.len().saturating_sub(lim);