feat: add inheritable-only field to filter processor-level targets (#5270)

* feat: add inheritable-only field to hide processor-level targets from listing

Processor-level targets like esp32, rp2040, etc. have flash-method set
so they leak through the existing heuristic filter in GetTargetSpecs and
appear in `tinygo targets` output. Add an "inheritable-only" JSON field
that is checked on raw JSON before inheritance resolution, preventing
these non-board targets from being listed while keeping them loadable
for direct builds and inheritance.

Ref: tinygo-org/tinygo#5178

* fix: prevent inheritable-only from propagating to child targets

The InheritableOnly bool field was propagating from parent to child
targets through overrideProperties, which would hide board targets
from GetTargetSpecs. Fix by preserving/restoring the field across
resolveInherits. Also simplify GetTargetSpecs by removing the raw
JSON pre-check workaround and remove nonexistent esp32c6 from tests.

* prevent build commands to use inheritable only targets

main.go

@@ -39,6 +39,8 @@ import (
 	"go.bug.st/serial/enumerator"
 )
 
+var errInheritableOnly = errors.New("target is inheritable-only, which means it cannot be used directly for building or flashing")
+
 // commandError is an error type to wrap os/exec.Command errors. This provides
 // some more information regarding what went wrong while running a command.
 type commandError struct {
@@ -140,6 +142,10 @@ func printCommand(cmd string, args ...string) {
 
 // Build compiles and links the given package and writes it to outpath.
 func Build(pkgName, outpath string, config *compileopts.Config) error {
+	if config.Target != nil && config.Target.InheritableOnly {
+		return errInheritableOnly
+	}
+
 	// Create a temporary directory for intermediary files.
 	tmpdir, err := os.MkdirTemp("", "tinygo")
 	if err != nil {
@@ -357,6 +363,10 @@ func Flash(pkgName, port, outpath string, options *compileopts.Options) error {
 		return err
 	}
 
+	if config.Target != nil && config.Target.InheritableOnly {
+		return errInheritableOnly
+	}
+
 	// determine the type of file to compile
 	var fileExt string
 
@@ -786,6 +796,10 @@ func Run(pkgName string, options *compileopts.Options, cmdArgs []string) error {
 		return err
 	}
 
+	if config.Target != nil && config.Target.InheritableOnly {
+		return errInheritableOnly
+	}
+
 	_, err = buildAndRun(pkgName, config, os.Stdout, cmdArgs, nil, 0, func(cmd *exec.Cmd, result builder.BuildResult) error {
 		return cmd.Run()
 	})