processor: introduce PostBuilder registry to replace hardcoded switch

Replace the large, duplicate switch statements in planPost and commitPlan
with a PostBuilder interface registered per file extension.

- PostBuilder interface: Plan(srcPath, ext) (postPlan, error) and
  Commit(plan, postDir, id, now) (*post.Post, []string, error)
- Per-type builders: txtBuilder, mdBuilder, imageBuilder, audioBuilder.
- Each builder self-registers via init() into a map[string]PostBuilder.
- Core processor loops are now extension-agnostic, satisfying OCP/DIP.
- All existing tests pass.

0 files changed, 0 insertions, 0 deletions