fix: return InvalidData instead of panicking on crafted wallet bytes

[zancas]

This PR stacks on #2749 and repairs one review finding. LightWallet::validate documents an io::Result whose error names the byte offset reached. The read path it wraps carried seven expect() calls that panic on crafted-but-reachable inputs. A validation entry point must not abort on the malformed files it exists to reject.

The seven sites now return InvalidData errors. They are: a pre-32 birthday above u32::MAX; account ids with the hardened bit set in the unified key store, the unified address map, and the transparent address map; a hardened transparent address index; a version 35 file whose key store vector holds no account 0; and a stored min_confirmations of zero. The three identical account id reads collapse into one read_account_id helper.

Five regression tests craft minimal wallet prefixes that reach the five shallow sites. Each test asserts that validate returns an error instead of panicking. The birthday and min_confirmations sites sit behind fields whose grammars are impractical to hand-craft. Those two sites receive the same mechanical repair without dedicated fixtures. The expect() at the account 0 lookup in read_v0 stays, because the map is built one line above it and the lookup cannot fail.

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