Capture the dialed destination at call ingress.

API changes (breaking — new public fields on non-exhaustive structs):
- SessionState and SessionSnapshot gain `initial_destination: Option<String>`,
  the destination of the first `Processing` dialplan line. Set once and never
  overwritten, so it survives later feature-context routing (transfer /
  execute_extension) that clobbers the last-wins `dialplan_to`. This is the
  reliable dialed number for public-profile calls where the
  Caller-Destination-Number CHANNEL_DATA field is absent.

Fixes:
- parse_processing_line now anchors the caller_id_number->destination boundary
  on the last `>->` (with a bare `->` fallback), so a free-form caller_id_name
  containing `->` is no longer mistaken for the boundary.

fslog:
- `search --session` now prints `dest=<number>`.