tidy-calllog: one spelling for the name of a UTC partition

[dinstein]

Nightly tidy pass over internal/calllog (2026-08-10 02:47 round).

• Pass A — the day-partition format was a named constant used six
  times in capacity.go and a bare "2006-01-02" in four other files —
  including two length checks spelled len("2006-01-02") — plus two
  outside the package, in the CLI and the control plane, rendering a prune
  cutoff into the very response that carries the partition names removed.

  The format decides three things at once: which directory a record is
  written into, which names the pruner validates before `os.RemoveAll`,
  and which ones a reader walks. They agreed by having been typed the same
  way. `DayLayout` moves to `types.go` beside the other frozen on-disk
  names, for the reason `PressurePolicy` is already there.
  

• Pass B — nothing cleared the bar.

• Pass C — nothing to reconcile.

Mutation-checked: shortening DayLayout to "2006-01" fails four tests.
Before this change the same edit moved capacity.go alone — the shape where
a pruner stops recognising the directories the writer is still filling.

No behaviour change; every spelling had the same value. Verification:
make ci green.