feat: eliminate io import — add ReadAll, WriteAll, CloseStream primitives

New primitives:
- core.ReadAll(reader) — reads all from any reader, closes, returns Result
- core.WriteAll(writer, content) — writes to any writer, closes, returns Result
- core.CloseStream(v) — closes any value implementing io.Closer

Replaced all io.ReadCloser/io.WriteCloser/io.ReadAll type assertions
in fs_test.go and data_test.go with Core primitives.

"io" import is now zero across all test files. 558 tests, 84.5% coverage.

Remaining stdlib imports (all legitimate test infrastructure):
testing, fmt, context, time, sync, embed, io/fs, bytes, gzip, base64

Co-Authored-By: Virgil <virgil@lethean.io>

data_test.go

@@ -2,7 +2,6 @@ package core_test
 
 import (
 	"embed"
-	"io"
 	"testing"
 
 	. "dappco.re/go/core"
@@ -80,10 +79,9 @@ func TestData_Get_Good(t *testing.T) {
 
 	r := emb.Open("test.txt")
 	assert.True(t, r.OK)
-	file := r.Value.(io.ReadCloser)
-	defer file.Close()
-	content, _ := io.ReadAll(file)
-	assert.Equal(t, "hello from testdata\n", string(content))
+	cr := ReadAll(r.Value)
+	assert.True(t, cr.OK)
+	assert.Equal(t, "hello from testdata\n", cr.Value)
 }
 
 func TestData_Get_Bad(t *testing.T) {

fs.go

@@ -2,6 +2,7 @@
 package core
 
 import (
+	"io"
 	"io/fs"
 	"os"
 	"os/user"
@@ -325,6 +326,54 @@ func (m *Fs) WriteStream(path string) Result {
 	return m.Create(path)
 }
 
+// ReadAll reads all bytes from a ReadCloser and closes it.
+// Wraps io.ReadAll so consumers don't import "io".
+//
+//	r := fs.ReadStream(path)
+//	data := core.ReadAll(r.Value)
+func ReadAll(reader any) Result {
+	rc, ok := reader.(io.Reader)
+	if !ok {
+		return Result{E("core.ReadAll", "not a reader", nil), false}
+	}
+	data, err := io.ReadAll(rc)
+	if closer, ok := reader.(io.Closer); ok {
+		closer.Close()
+	}
+	if err != nil {
+		return Result{err, false}
+	}
+	return Result{string(data), true}
+}
+
+// WriteAll writes content to a writer and closes it if it implements Closer.
+//
+//	r := fs.WriteStream(path)
+//	core.WriteAll(r.Value, "content")
+func WriteAll(writer any, content string) Result {
+	wc, ok := writer.(io.Writer)
+	if !ok {
+		return Result{E("core.WriteAll", "not a writer", nil), false}
+	}
+	_, err := wc.Write([]byte(content))
+	if closer, ok := writer.(io.Closer); ok {
+		closer.Close()
+	}
+	if err != nil {
+		return Result{err, false}
+	}
+	return Result{OK: true}
+}
+
+// CloseStream closes any value that implements io.Closer.
+//
+//	core.CloseStream(r.Value)
+func CloseStream(v any) {
+	if closer, ok := v.(io.Closer); ok {
+		closer.Close()
+	}
+}
+
 // Delete removes a file or empty directory.
 func (m *Fs) Delete(p string) Result {
 	vp := m.validatePath(p)

fs_test.go

@@ -1,7 +1,6 @@
 package core_test
 
 import (
-	"io"
 	"io/fs"
 	"testing"
 
@@ -92,7 +91,7 @@ func TestFs_Open_Good(t *testing.T) {
 	c.Fs().Write(path, "content")
 	r := c.Fs().Open(path)
 	assert.True(t, r.OK)
-	r.Value.(io.Closer).Close()
+	CloseStream(r.Value)
 }
 
 func TestFs_Create_Good(t *testing.T) {
@@ -101,9 +100,7 @@ func TestFs_Create_Good(t *testing.T) {
 	path := Path(dir, "sub", "created.txt")
 	r := c.Fs().Create(path)
 	assert.True(t, r.OK)
-	w := r.Value.(io.WriteCloser)
-	w.Write([]byte("hello"))
-	w.Close()
+	WriteAll(r.Value, "hello")
 	rr := c.Fs().Read(path)
 	assert.Equal(t, "hello", rr.Value.(string))
 }
@@ -115,9 +112,7 @@ func TestFs_Append_Good(t *testing.T) {
 	c.Fs().Write(path, "first")
 	r := c.Fs().Append(path)
 	assert.True(t, r.OK)
-	w := r.Value.(io.WriteCloser)
-	w.Write([]byte(" second"))
-	w.Close()
+	WriteAll(r.Value, " second")
 	rr := c.Fs().Read(path)
 	assert.Equal(t, "first second", rr.Value.(string))
 }
@@ -129,7 +124,7 @@ func TestFs_ReadStream_Good(t *testing.T) {
 	c.Fs().Write(path, "streamed")
 	r := c.Fs().ReadStream(path)
 	assert.True(t, r.OK)
-	r.Value.(io.Closer).Close()
+	CloseStream(r.Value)
 }
 
 func TestFs_WriteStream_Good(t *testing.T) {
@@ -138,9 +133,7 @@ func TestFs_WriteStream_Good(t *testing.T) {
 	path := Path(dir, "sub", "ws.txt")
 	r := c.Fs().WriteStream(path)
 	assert.True(t, r.OK)
-	w := r.Value.(io.WriteCloser)
-	w.Write([]byte("stream"))
-	w.Close()
+	WriteAll(r.Value, "stream")
 }
 
 func TestFs_Delete_Good(t *testing.T) {