orchestrator: split capture_result into named phases

The tagged-line parser had grown to ~290 lines doing prefix parse,
region/name split, tail key collection, sz->hi normalization, cross-key
and pos validation, static-VAS bounds, and slot population all in one
function. Extract four phases — parse_region_field, parse_result_tail
(filling a named struct parsed_tail), result_vas_ok, and append_result —
leaving capture_result as a short pipeline.

Pure restructuring: every check and its order are preserved, so the wire
parsing behaviour is unchanged (covered by the existing capture_result
unit tests and fuzz harness).

Author: bcoles