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fix(immut/sorted_map): rebuild filter results with join and concat

`filter` recombined the independently filtered subtrees with a single
`balance` step when it kept the node, or with `glue` when it dropped it.
Both assume the two sides are within one rotation of balance, but the
filtered sides can differ arbitrarily in size, so the result could break
the size-balance invariant: filtering 2000 entries down to one side of the
root plus one key on the other left a node with subtree sizes (1, 249).

Recombine with `join` when the node is kept and with a new private
`concat` (split_min of the right side, then `join`) when it is dropped,
as @immut/sorted_set's filter does. The predicate is still called in the
same order.

Signed-off-by: Codex CLI <codex@openai.com>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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