feat: kicad_tools headless CLI

One merged node-WASM CLI (wasm/tools, 26.5MB) supersedes sym_convert +
pcb_convert: --convert-lib (absolutizes paths — fixes the legacy plugin's
silent empty output on relative paths), --lint (now full-parses .kicad_pcb
via the linked pcbnew parser), --erc, --netlist, --bom, --plot, --drc.
Registered as the only headless CLI app; ASYNCIFY=0 CLIs skip the
nanosleep→Asyncify yield shim and exit via _exit (dieted static dtors trap).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ADzCSeN3Q9DJL3YW2FZTXB

wasm/cli/pcb_convert_stubs.cpp

@@ -0,0 +1,15 @@
+/*
+ * pcb_convert link-order diet stubs (mirror of sym_convert_stubs.cpp).
+ *
+ * Listed before the kiface objects in the pcb_convert link so
+ * --allow-multiple-definition resolves duplicated definitions to these
+ * (first definition wins). Grows as the diet surfaces link/runtime needs;
+ * function references left dangling by the CMake prune are covered by
+ * -sERROR_ON_UNDEFINED_SYMBOLS=0 (they become aborting JS imports), so only
+ * data/typeinfo needs and behavior overrides belong here.
+ *
+ * Currently empty: the --drc path needs no overrides — board *save* and the
+ * plot/export surfaces are pruned outright, and Kiface() is deliberately left
+ * undefined so a stray call aborts loudly instead of wandering on a null
+ * settings object.
+ */