Addresses review finding C3.
Before this commit, bintrieFlatReader.Account returned (nil, nil) when
both the BasicData and CodeHash leaves were absent from the flat state.
multiStateReader.Account treats (nil, nil) as "confirmed absent" and
short-circuits — the trie reader never runs. This silently hid every
corruption mode the other A-commits are fixing (C1 mid-stem resume
loss, C2 disk-layer shape mismatch, in-transition stale data, etc.):
the flat state said "not present" and nobody checked.
Fix: introduce errBintrieFlatStateMiss as a local sentinel. When both
leaves are absent, the flat reader returns (nil, errBintrieFlatStateMiss)
instead of (nil, nil). The multiStateReader falls through on any
non-nil error, so the trie reader now runs and serves as the
authoritative gatekeeper. If the flat state genuinely has no data (and
the trie reader also returns nil), the end result is the same — but
any case where the flat state is wrong and the trie is right is now
caught by the fallthrough.
Same treatment for Storage: absent blob returns errBintrieFlatStateMiss.
Known limitation: BinaryTrie.GetAccount does not verify stem membership
(a characteristic of verkle-style tries where non-membership proofs are
handled externally). A truly non-existent account returns the closest
stem's data, not nil. The TestBintrieFlatReaderMissingAccountSentinel
test therefore verifies the flat reader's sentinel in isolation rather
than the end-to-end multiStateReader result.
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