084 task template reapply: Flow PT REST finding

Q What does setting task_template on a Shot that already has Tasks do: add, duplicate, remove, or nothing? Is there a mode?

Endpoint PUT /entity/shots/<id> ; POST /entity/_batch ; POST /entity/tasks/_search

Docs claim Silent. /spec.json mentions task_template twice, both in sample records, and TaskTemplate never.

Actual

tt1: same@Animation, moved@Character FX, only1@Comp      tt2: same@Animation, moved@Comp, only2@FX
Shot A created with tt1; `same` set to ip; one hand-made Task added   -> 4 Tasks
PUT task_template tt2        -> 200, 7 Tasks: + only2, + moved@Comp, + same@Animation (status wtg)
                                 the tt1 Tasks stay, `same` keeps ip, the hand-made one stays
PUT task_template tt2 again  -> 200, 7 Tasks, nothing added
PUT task_template tt1        -> 200, 7 Tasks, nothing added
PUT task_template null       -> 200, 7 Tasks, Shot.task_template null
Shot B bare, hand-made same@Animation, PUT tt1 -> 4 Tasks: the hand-made `same` and a second `same`
Shot C created with tt1, _batch update to tt2  -> 6 Tasks, as the PUT
Shot D created with tt1, generated only1 deleted, PUT null, PUT tt1 -> only1 re-created, 3 Tasks
Shot E created with tt1, generated only1 deleted, PUT tt1 unchanged -> 200, 2 Tasks, nothing made
PUT {"task_template": tt2, "task_template_mode": "replace"}
  -> 400 code 103 "API update() Shot.task_template_mode doesn't exist." (nothing applied)
Shot.task_template schema properties: default_value, summary_default, valid_types

Teaches

on the Shot already	the new template's task	result
nothing	any	created
a Task whose `template_task` is this template task	the same	not re-created; re-synced to it, fields and edges (probe 102)
a Task made from another template, same `content` and `step`	this one	created: a duplicate
a Task made from another template, same `content`, other `step`	this one	created
a hand-made Task, same `content` and `step`	this one	created: a duplicate
a Task from the old template the new one lacks	none	kept

The match key is template_task, not content and step. A template task is not re-created only when a Task on the entity already points at it. Delete that Task and the next apply re-creates it.
No Task is ever removed. Switching templates, and clearing the field, leave every Task in place, statuses and all. A replace means the caller deletes the Tasks itself (probe 089 for what that breaks).
It does not only add. The Tasks linked to the new template have their fields overwritten and the edges between them reset to the template's (probes 096, 102 correct this entry's first reading).
The apply runs only when the value changes. Sending the stored value again added nothing on Shot E although a Task was missing; clear the field first, then set it, as Shot D did.
There is no mode. No body key, schema property or documented parameter selects keep, replace or merge. To merge by content and step, point each matching Task's template_task at its template task first, then write task_template (recipe 015_apply_task_template_without_duplicates).