fix(node): an uploaded file records who sent it

`_register_uploader` walked the index for the entry it had just written, at a
moment when no such entry can exist: the file was a `.part` until the rename on
the line above, which is not indexable, and the watchdog that will index it
debounces for two seconds and then hashes. The walk matched nothing, silently,
so every uploaded file in every group was owned by nobody — and `file_delete`
refuses a caller with no admin authority when the entry records no uploader, so
a member could not delete what they had just sent. MESHBAY_DESIGN.md §5.4
grants that to any non-revoked device of the uploading account.

The record is now written when the last chunk lands (`indexer.record_upload`)
and the entry is stamped from it in `_hash_or_cached`, the one funnel every
entry passes through — initial scan, watchdog, reconcile and replug alike. It
lives in the index cache rather than on the entry alone, because the index is
rebuilt from disk at every start and an owner the node forgets on restart is a
right quietly taken away. It is validated against a live `stat()`, so whatever
later occupies that path inherits nothing; and `_rescan_root`'s carry-over no
longer copies over it, or memory would beat the durable record.

§5.4 also claimed ownership was *provable* — a transcript the uploader signs,
stored with the entry. No such signature has ever existed; `meshbay:upload:v1`
in the code is the groupbox purpose that seals the envelope. The section now
states what the code does, and the transcript is an open item in §15.3.

`test_upload_attribution.py` drives the real handler and a real indexer across
that seam. Against the previous source its two positive cases fail on the
property, not on a missing method — an upload, then a rebuild from disk, then
a different file at the same path inheriting nothing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UMxEQadpzPkYLFf5CYKhpW

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