feat(holder): cache and reuse proofs for unchanged credentials (#426)

[CodingBabe-1]

What does this PR do?

Fixes #426: a holder who already generated a valid proof for a credential no longer re-proves it needlessly when the underlying credential, its parameters, and the circuit are unchanged.

Proof generation is the most expensive step in the holder flow (server-side Noir witness + browser WASM UltraHonk). This PR caches generated proofs locally, keyed by the exact inputs that determine a proof — commitment, claim parameters, and circuit/VK version — and reuses a valid, matching prior proof instead of regenerating it. Reuse is surfaced in the UI with a clear "using existing proof" banner so it's visible rather than a silent optimisation.

How the cache works

Every proof is a pure function of:

• the credential commitment (derived from type, value, salt, issuer),
• the claim parameters baked into the credential (thresholds / restricted list),
• the circuit / VK version it was generated with.

lib/proof-cache.ts derives a deterministic buildProofKey from exactly these three, stores proofs locally (JSON-safe byte arrays), and:

• reuses a cached proof only when the key matches and the on-chain record is still valid,
• invalidates on expiry (local provedAt + TTL window, and a live is_verified read),
• invalidates on revocation (the on-chain record no longer validates),
• invalidates on VK version change — the VK version is itself derived from the actual compiled circuit bytecode (resolveVkVersion), so bumping a circuit or toolchain automatically invalidates every cached proof,
• invalidates on param change (any threshold / restricted change produces a new key).

Holder page integration

• ProofFlow checks the cache before doing any expensive work; on a hit it jumps straight to the generated state with a "Using existing proof — params unchanged" banner. Fresh proofs are saved to the cache, and a successful on-chain submission marks the entry as proved (with its TTL) so it stays reusable until it expires.
• BatchProofFlow does the same per credential and shows a per-row "using existing proof" badge; successful batch submission records every reused/fresh proof's cache entry.
• Removing a credential also clears its cached proofs.

Acceptance checklist

• A valid, matching prior proof is reused instead of regenerated (unit-tested).
• Correctly invalidated on expiry, revocation, VK version change, or param change (unit-tested).
• Reuse is visible in the UI ("using existing proof").
• Existing self-service flows unchanged; proving still works exactly as before when no cached proof applies.

Type of change

• Bug fix
• New feature / credential type
• Refactor / cleanup
• Docs
• CI / tooling

Checklist

• cargo test passes (contracts) — no contract changes
• pnpm tsc --noEmit passes (frontend)
• pnpm build passes (frontend)
• pnpm test passes (frontend – includes issuer suite)
• Circuit changes: none (VK-version handling reads the existing shipped circuit bytecode)
• No NEXT_PUBLIC_ prefix on server-only env vars
• No identity fields stored or logged after KYC provider call

Notes for reviewers

• Reuse is keyed on the credential as stored in the local wallet, so an on-chain is_verified read is only attempted when the local record already says the proof is currently valid; if contracts aren't reachable it falls back to the local expiry check. This means the cache is never trusted blindly — a mismatch in any dimension triggers a fresh proof.
• The circuit bytecode hash is computed with crypto.subtle, which is available in the app's secure-context browsers.
• New test tooling (vitest config / pnpm test) is shared across the frontend workspace; the standalone @stellarcred/issuer suite keeps its own node-environment runner.

closes #426

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