feat(rpiv-pi): disk-corroborated basename fallback for the artifact collectors

Regression: run 2026-08-21_12-15-19-ec5e, code (phase 1/7). The agent wrote
its elaboration to the correct .rpiv/artifacts/elaborations/ path but
announced it in prose as `.elaborations/<file>.md` — directory prefix
mangled. The full-path scan missed, the stage fataled, and a verified-green
38KB artifact was orphaned: the transcript (the report of the work) was the
only channel checked, never the filesystem (the work itself).

On a full-path miss both collectors now scan the transcript for bare
`<file>.md` tokens (tempered — an elided `...__phase-N.md` still never
resolves) and accept a candidate ONLY when it names exactly one existing
file under .rpiv/artifacts/ (the collector's own bucket for the
bucket-narrowed form). The announcement contract is not weakened: the
agent's own text still drives collection, disk existence corroborates it —
a stray prose mention (README.md), an ambiguous basename (same file in two
buckets), or a wrong-bucket hit refuses, and with no unique resolution the
original fatal stands. Eight tests pin the recovery and every refusal shape.