cdk-axum: Proof-of-Reserves /audit endpoint + experimental-features info page

[mvuk]

cdk-axum: serve a Proof-of-Reserves attestation + experimental-features info

Experimental / discussion-first (opened as draft). This supports a Bark
(Ark)-backed mint; feedback on shape and whether it belongs behind a feature flag
is very welcome.

Adds mint-side serving for a Proof-of-Reserves attestation and a small info-page
surface. Scoped entirely to crates/cdk-axum/.

Changes

• GET /audit/latest.json — serve a file-backed PoR bundle (CORS-open).
• Info page — render the live attestation (reserve / block height + bundle link),
  framed as a lower bound ("Lower bound across N live Bark VTXO(s) as of block X");
  an "Experimental features" section (payjoin board, on-chain send, PoR) below the NUTs.
• GET /icon.png — serve a file-backed mint icon (CORS).

Notes

• The bundle is produced out-of-band by the payment processor; this PR only serves it.
• PoR framing is intentionally a lower bound — never a completeness claim.
• Commits are DCO Signed-off-by.

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