Avoid patching v-model handler when possible

Editor assessment

The subject states the optimization and its condition exactly — the v-model handler is patched only when necessary — so the header communicates the change, its mechanism, and the redundant work it avoids without the diff, and the path-style scope compiler-core/v-model precisely names both the compiler section and the directive. feat is the right classification. Hygiene is clean: imperative mood, no trailing period, sensible length, no ticket junk. No body is present, tolerable for a contained optimization, and no footer is required. Clear, accurate, and publishable: a good example.