Add impression_id to TMPX or Offer for pixel deduplication

[bokelley]

Context

The AXE system included an impression_id generated at request time for exposure deduplication. TMPX (added in #2079) provides per-user exposure tracking via encrypted tokens but does not include a buyer-generated impression ID.

Without an impression_id, the buyer's pixel must generate its own ID at fire time and correlate it back to the TMPX token. If the impression_id were inside the TMPX plaintext, the master gets identity + impression context in one decryption.

Options

1. Add ext bytes to TMPX plaintext — variable-length buyer-defined data after identity entries. Tighter on space.
2. Dedicated impression_id field on the Offer — flows as a separate URL parameter alongside {TMPX}. No size pressure.
3. Both — ext in TMPX for identity-correlated metadata, Offer field for everything else.

Raised by

@BaiyuScope3 in #2079 review — noting that the existing AXEM implementation returns an impression_id generated at RT.

🤖 Generated with Claude Code

[bokelley]

think we decided on TMPY macro for context match instead