Add rebalance first slice (3d-7)

First vertical slice for portfolio rebalancing. Domain gains
RebalancePolicy: target-allocation validation (non-empty, per-code
shares positive two-decimal, sum exactly 100); diff computation over
the union of target and held codes (held without target = full exit)
producing BUY amount diffs and SELL unit diffs priced at the current
valuation NAV, clamped to available units, with no orders when a
weight already balances; and the deterministic idempotency key set
(rebalance:{plan}:{date}:{code} plus confirm/redeem counterparts).

Rebalance plans persist as rebalance_plans + rebalance_targets with
rebalance_idempotencies under the shared replay/conflict contract.
ExecuteRebalancePlan snapshots cash plus holdings valued at each
position's latest valuation NAV, then walks the diffs SELL-first
through the same Create/Confirm pipeline used by manual orders,
redemptions and SIP (nothing bypasses confirmation; a held code
without a valuation is skipped with an explicit marker; insufficient
cash/units fail the period visibly instead of silently trimming).

API exposes POST/GET /funds/{id}/rebalance/plans and POST
.../plans/{id}/execute. The frontend adds an 08/REBALANCE panel with
target editing (two codes, sum must equal 100), a plan list with a
manual execute button and post-run outcome rows; executing refreshes
orders/positions so the new orders are immediately visible. Domain
tests cover split validation, diff orders, exit/clamp logic, key
determinism; API tests cover create/execute end-to-end; Web tests
cover decoding and stale-execution protection.

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