docs: library management feature design notes

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/features/libraries/0001-library-management.md

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+# How KiCad manages libraries — embedded vs. referenced
+
+## Question
+
+If I create a design that uses some component parts and open it in a brand-new
+KiCad build (no libraries installed), are the parts burned into the file, or do
+I still need the original libraries downloaded?
+
+## Short answer
+
+Mostly **yes — KiCad caches/embeds the part definitions into the design file
+itself**, so existing files open and render without the original libraries. The
+details differ between schematic symbols, PCB footprints, and 3D models.
+
+This matters for the WASM build: it determines whether we need to ship library
+files to the browser to view/edit a given file.
+
+## Schematic symbols (`.kicad_sch`)
+
+KiCad 6+ writes a `(lib_symbols ...)` block near the top of every schematic
+file. This is a **full embedded copy** of every symbol used on the sheet — pins,
+graphics, fields, the lot. On open:
+
+- KiCad renders and edits from this **embedded cache**, not from the original
+  library.
+- Opening on a fresh install with **zero libraries** works fine — the parts are
+  effectively burned in.
+- Each symbol keeps a `lib_id` (e.g. `Device:R`) which is just a *name link*
+  back to the source library. That link is only used when you explicitly run
+  **Tools → Update Symbols from Library**, or when placing a *new* instance.
+
+In KiCad 5 and earlier the same caching was done via an external
+`<project>-cache.lib` sidecar file next to the project — same idea, just not
+inline.
+
+## PCB footprints (`.kicad_pcb`)
+
+Footprints are **always fully embedded** — no caching ambiguity. Every footprint
+instance carries its complete geometry (pads, courtyard, silkscreen). A
+`.kicad_pcb` is entirely self-contained; the `.pretty` footprint libraries are
+never needed to open or edit a board.
+
+## 3D models — the exception
+
+By default, 3D models (`.step` / `.wrl`) are **not embedded**. They are stored as
+**file path references**, often via env vars like `${KICAD8_3DMODEL_DIR}`. On a
+fresh machine the board still opens, but the 3D viewer shows nothing/placeholders
+unless those model files exist.
+
+KiCad 7+ added an **Embedded Files** feature (Board / Schematic Setup → Embedded
+Files) to explicitly bundle 3D models, fonts, etc. into the design file for true
+portability.
+
+## Implications for kicad-wasm
+
+A `.kicad_sch` or `.kicad_pcb` opened in the browser build will **render and edit
+correctly without downloading any libraries**, because symbol/footprint
+definitions travel inside the file. Libraries are only required for:
+
+1. **Placing new parts** — the symbol/footprint chooser needs library tables plus
+   the `.kicad_sym` / `.pretty` files.
+2. **3D rendering** — unless models are embedded.
+3. **"Update from Library"** workflows.
+
+So for a viewer, or even a basic editor of *existing* files, we can skip shipping
+libraries entirely.