reorganize: consolidate demo directories

- Consolidate _demo, _pydemo, _embdemo into single _demo directory structure
- Organize demos by language: _demo/{go,py,c,embed}/
- Categorize demos based on imports:
- Python library demos (py imports) → _demo/py/
- C/C++ library demos (c/cpp imports) → _demo/c/
- Go-specific demos → _demo/go/
- Embedded demos → _demo/embed/
- Move C-related demos (asm*, cabi*, cgo*, linkname, targetsbuild) from go/ to c/
- Update all path references in README.md and GitHub workflows
- Improve demo organization and navigation as requested in #1256

🤖 Generated with [Claude Code](https://claude.ai/code)

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

_demo/c/asmfullcall/asmfullcall_linux.go

@@ -0,0 +1,30 @@
+//go:build linux && amd64
+
+package main
+
+import "unsafe"
+
+func verify() {
+	// 0 output & 0 input
+	asmFull("nop", nil)
+
+	// 0 output & 1 input with memory address
+	addr := uintptr(unsafe.Pointer(&testVar))
+	asmFull("movq {value}, ({addr})", map[string]any{
+		"addr":  addr,
+		"value": 43,
+	})
+	check(43, testVar)
+
+	// 1 output & 1 input
+	res1 := asmFull("movq {value}, {}", map[string]any{
+		"value": 41,
+	})
+	check(41, int(res1))
+
+	res2 := asmFull("leaq ({a},{b}), {}", map[string]any{
+		"a": 25,
+		"b": 17,
+	})
+	check(42, int(res2))
+}