Phase 2: image register --kernel-ref resolves through the catalog

`banger image register --kernel-ref <name>` now substitutes for the
--kernel/--initrd/--modules triple. The daemon looks the name up via
kernelcat.ReadLocal under d.layout.KernelsDir, populates the three
paths from the resolved entry, then continues through the existing
validate/persist flow unchanged.

Passing both --kernel-ref and any of --kernel/--initrd/--modules is
rejected — at the CLI layer (before starting the daemon) and
defensively at the RPC layer. A missing catalog entry produces a clear
"run 'banger kernel list'" message.

Once registered, the image stores the resolved absolute paths, so
deleting the catalog entry later does not invalidate already-registered
images — managed image build still copies the kernel into its artifact
dir per imagemgr.StageBootArtifacts.

Tests cover: resolution success (absolute KernelPath populated from
catalog), mutual-exclusion rejection, and missing-entry error.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/api/types.go

@@ -259,6 +259,7 @@ type ImageRegisterParams struct {
 	KernelPath   string `json:"kernel_path,omitempty"`
 	InitrdPath   string `json:"initrd_path,omitempty"`
 	ModulesDir   string `json:"modules_dir,omitempty"`
+	KernelRef    string `json:"kernel_ref,omitempty"`
 	Docker       bool   `json:"docker,omitempty"`
 }