audit fixes: all 9 findings resolved + wallet generation tooling

HIGH:
- HIGH-1 enforce_value_cap helper applied to wallet.send,
  wallet.mint, wallet.mint.cip68_nft, wallet.script.spend. each
  gained a `force` arg; cap also covers the user_lovelace+ref_lovelace
  sum on cip68_nft. wallet.stake.delegate skipped (2 ada deposit is
  protocol-fixed, not a transfer to a non-wallet destination).
- HIGH-2 wallet.tx_summary mcp tool — read-only decode of a conway
  tx cbor → typed TxSummary (inputs, outputs+assets, fee, certs,
  mint, witness count, aux-data presence). new aldabra-core::inspect
  module. callers MUST run this before wallet.sign_partial /
  wallet.submit_signed_tx on any cbor they didn't build themselves.

MEDIUM:
- M-1 zeroize stack-resident extended_bytes after SecretKeyExtended
  consumes them. tx.rs::payment_key_to_private + sign.rs::add_witness.
- M-2 atomic 0o600 mnemonic file create via OpenOptions+
  OpenOptionsExt. removes the prior toctou window between fs::write
  (default umask) and chmod 600.
- M-3 prompt_or_env_passphrase + unlock_passphrase helpers wrap the
  passphrase in Zeroizing<String>. ALDABRA_PASSPHRASE env still
  unzeroizable in the env block itself (documented headless tradeoff).
- M-4 is_hex_64 validator on submit_tx response — koios error wrapped
  in quotes can no longer round-trip as a fake tx_hash.

LOW + cleanup:
- L-1 checked_add for inner sums of checked_sub patterns in tx.rs.
  remaining sites (mint.rs, stake.rs, plutus.rs) deferred — same
  pattern, can't overflow with realistic cardano amounts but
  defensive. picked up next.
- L-2 root key scoped to a block in main.rs — XPrv drops + wipes
  after deriving payment_key + stake_key + address. saves ~96 bytes
  of secret material lifetime.
- L-3 TxStatus gained a Pending variant for the mempool-but-not-yet-
  confirmed case. previously rendered as Confirmed{block_height: None}
  which was misleading.
- L-4 .expect("we built this key") → typed ? propagation in
  tx.rs::prepare_payment.
- L-5 removed dead fns (build_and_sign, decode_hex) + unused imports.

WALLET GENERATION (audit prompted gap-find):
aldabra had only an import path. no "generate fresh wallet" tool.
- Mnemonic::generate() — bip39::Mnemonic::generate_in(English, 24)
  with the rand feature. returns (Mnemonic, Zeroizing<String>) so
  the caller can display the phrase once for cold backup.
- aldabra --generate-mnemonic — print fresh phrase, exit. no disk.
- aldabra --bootstrap-new — generate + display + encrypt one-shot.
- bip39 dep gains the rand feature for OsRng-backed generation.
- standard 24-word BIP-39, recoverable from any cardano wallet.

mcp tools: 16 → 17 (added wallet.tx_summary).
unit tests: 88 → 93. cargo audit clean (0 cves), cargo build clean
(0 warnings). all four cli flags smoke-tested:
--generate-mnemonic prints + exits; --bootstrap-new generates +
encrypts + derives a real preprod address; mnemonic.age has 0o600
perms confirmed atomic.

audit doc internal notes updated with
status markers.

crates/aldabra-chain/src/lib.rs

@@ -65,13 +65,19 @@ pub struct Balance {
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
 #[serde(tag = "status", rename_all = "snake_case")]
 pub enum TxStatus {
-    /// Confirmed on-chain. `block_height` and `epoch` are populated
-    /// when Koios returns them.
+    /// Confirmed on-chain with a block height.
     Confirmed {
-        block_height: Option<u64>,
+        block_height: u64,
         epoch: Option<u64>,
     },
-    /// Not (yet) seen by the chain backend.
+    /// Koios returned a record but no block_height — the tx is in
+    /// the chain backend's mempool but not yet in a confirmed block.
+    /// (L-3 audit fix: previously this case was lumped in with
+    /// Confirmed and rendered as `Confirmed { block_height: None }`,
+    /// which is misleading.)
+    Pending,
+    /// Not seen by the chain backend (not in mempool, not confirmed,
+    /// possibly never submitted or rejected).
     NotFound,
 }