doc: add non-technical one-pager for the Dilithium PSBT testnet proof

[Oskii]

Summary

• Adds design/DILITHIUM_PSBT_PROOF.md, a one-page summary of the 2-of-3 Dilithium multisig spend proven on the public testnet, written for a reader who should not have to read a wire format specification to understand what was demonstrated.
• Ships the rendered DILITHIUM_PSBT_PROOF.pdf for circulation, plus the .pdf.css used to produce it so the layout is reproducible rather than hand-rebuilt. The markdown is the source of record.
• Covers what a PSBT is, what Dilithium could not do through one before, the two signing flows exercised (sequential hand-off and independent parallel signing then merge), the three negative tests, and why acceptance by unrelated miners makes the result independent rather than self-confirming.

Both transactions are linked to the public explorer, so every factual claim is checkable without running anything:

• Payment 1, block 215,455: 6fc71b62…0bc9
• Payment 2, block 215,469: 8d31a6cf…2ff5

Ordering note

Opened as a draft on purpose. The one-pager describes behaviour that is not on master yet: the implementation, the design doc's live-chain proof section, and the re-runnable proof script all arrive with the implementation pull request. The footer therefore points only at design/DILITHIUM_PSBT_DESIGN.md, which does exist, rather than at the section and script that do not. This should merge after the implementation lands, at which point the footer can be pointed at the proof script directly.

Test plan

• design/DILITHIUM_PSBT_PROOF.pdf opens and is a single page
• Both explorer links resolve and show a witness carrying two 2,421-byte Dilithium signatures plus one empty slot
• The empty slot sits in a different position in each of the two transactions, since a different pair signed each
• The stated address matches the one both transactions spend from
• Regenerating the PDF from the markdown and CSS reproduces the committed layout