Return recoverable errors for duplicate review tree keys

[jmqd]

TUI tree construction can assert-panic when splitter/cache data produces duplicate block lookup keys. Audit refs: trueflow/src/commands/tui.rs:3218, trueflow/src/tree.rs:232, trueflow/src/commands/review.rs:334, trueflow/src/block.rs:368, trueflow/src/tree.rs:532. Triggers: dynamic child expansion, corrupt scan cache, or splitter regression. Direction: de-duplicate or validate before insertion and surface a recoverable diagnostic instead of panicking.