v0.1.1 — a moved shape is never an empty listing

Exit codes change for a page the parser cannot read. v0.1.0 treated a
listing record with no usable UniqueId or no Seller as advertising, so
a moved payload shape read as an EMPTY page: exit 4 on page 1 ("nothing
matched"), and a complete run with exit 0 when it happened on a later
page. It now ends as parser_found_nothing: exit 5 when nothing was read,
exit 6 (partial, rows kept) when earlier pages were. A pipeline that
treated exit 4 as "empty search" was being told something false.

Fixes from a third-party audit of v0.1.0, each reproduced before changing
anything and each pinned by a check that a planted fault turns red.

Fixed

• A record is sponsored only on a POSITIVE marker (MediaZeroKm or
  AdvertisementLink). Measured across every capture: 3 of 3 sponsored
  tiles carry both, 0 of 3,386 listings carry either. A listing that cannot
  be read is counted in the sidecar's new malformed_records and logged.
• Core columns that fall below their coverage floor are recorded in the
  sidecar (columns_below_floor), not only logged, and the canary fails on
  them and on any malformed record.
• Ad mode: the sidecar's query identifies the SET of adverts asked for
  (ads_sha256, order-independent) and lists them in ads_requested.
  v0.1.0 recorded only their number, so diff_runs.py compared two
  different lists of one length.
• diff_runs.py refuses two search runs of different depth
  (pages_requested): the extra pages read as listings the market added.

Changed

• A skipped canary run says so at the top of its summary page, and the
  README says the badge is green because the canary skips: this repository
  has no proxy secret set.