Vault module: WebAuthn-unlocked encrypted seed vault

[truthixify]

Extra labels: vault, feature, security
Tier: L (5-7 days) | Type: feature

Context. src/vault/ exists as a stub. Users want to store their STEALTH_SIGNING_MESSAGE derivation seed encrypted at rest and unlock with a WebAuthn touch. Removes the "have to sign every session" friction.

Scope.

• Vault v1: AES-GCM encrypted blob, key derived from a WebAuthn signature.
• Unlock produces the stealth signing seed in-memory only (no localStorage of plaintext).
• Wipe on tab close.
• Cross-tab lock coordination (BroadcastChannel).

Acceptance.

• Threat-model doc'd: what protects the vault, what doesn't
• Round-trip test: create → lock → unlock recovers identical seed
• Explicit non-goal: not a wallet, not a key manager for custody

Files. src/vault/webauthn.ts (new), src/vault/aes.ts (new), src/vault/README.md.

[Husten150]

@Husten150 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

i will like to apply on this issues please

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