python: verify the two gate facts (frame reader, KeyboardInterrupt) #1384

Created with Claude Code from the spec in #1341.

Parent

Sub-issue of #1341. Ticket 3 of 14.

What to build

Two written findings, each with the probe code that proved it, committed under the Python workspace's docs. The spec (#1341, Further Notes) names two facts the design assumes but nobody has verified:

1. vitaminc-aead-frame exposes a reader the extension can use to walk a stored frame and recover the value kind without decrypting. The decrypt preflight (DECRYPT_FOREIGN_ROW, DECRYPT_MALFORMED) depends on this.
2. With the GIL released around a blocking engine call, a KeyboardInterrupt lands only after the detached call returns, and an early-dropped Pending future wipes its buffers. The sync-and-GIL contract depends on this.

If either is false, draft the amendment to #1341 as a comment on #1341 in this ticket and name which downstream tickets change (the decrypt ticket, the concurrency ticket).

Acceptance criteria

• A findings file for each fact: what was probed, on which commit of the dependency, the result, and the probe code or a pointer to it.
• The probes are either committed as tests in the engine crate or recorded as throwaway with their output pasted.
• If a fact is false, a comment on Python SDK: core call over psycopg, on EQL v4 and Stack Encrypt #1341 states the amendment and the affected tickets.
• The decrypt and concurrency tickets are updated (comment) with "gate facts confirmed" or the amendment.