fix(trade): keep dimnames on the all-zero .balance_matrix() path

[lbm364dl]

What was wrong

.balance_matrix() (R/bilateral_trade.R) short-circuits when the balanced
export and import targets are all zero:

if (sum(exports) == 0 && sum(imports) == 0) {
  return(matrix(0, nrow = n, ncol = n))
}

That return carries no dimnames, while the normal return path builds its result
with dimnames = dimnames(trade_matrix). The documented contract for these
matrices (R/bilateral_trade.R:26) is name indexing — "the value at
m["A", "B"] is the trade in tonnes" — so the zero path quietly violated it.
It is benign today only because both consumers short-circuit on zero content
before indexing, which is exactly the accident worth pinning.

Classification: mechanical — a contract/crash fix with no defensible
alternative and no numeric effect (the returned values are all zero either
way).

What changed

• The zero path now sets dimnames = dimnames(trade_matrix), the same way the
  normal path does.
• New test in tests/testthat/test_bilateral_trade.R:
  .balance_matrix keeps dimnames when all trade is zero — feeds an all-zero
  group with named dimensions through and asserts both dimnames(result) and
  result["1", "2"].

The pre-existing .balance_matrix returns zero matrix when all trade is zero
test passes an unnamed input matrix, so it keeps passing unchanged.

Verification

Failing before the fix (source change stashed, new test present):

── Failed tests ────────────────────────────────────────────────────────────────
Failure ('test_bilateral_trade.R:669:3'): .balance_matrix keeps dimnames when all trade is zero
dimnames(result) (`actual`) not equal to list(codes, codes) (`expected`).

`actual` is NULL
`expected` is a list

Error ('test_bilateral_trade.R:670:3'): .balance_matrix keeps dimnames when all trade is zero
Error in `result["1", "2"]`: no 'dimnames' attribute for array

[ FAIL 2 | WARN 0 | SKIP 0 | PASS 87 ]

Passing after the fix:

✔ |         89 | bilateral_trade
[ FAIL 0 | WARN 0 | SKIP 0 | PASS 89 ]

Also ran air format . (no further changes to other files) and lintr::lint()
on both changed files — no lints. No roxygen changed, so no document() run
was needed; no NEWS.md entry, since no published value changes.

Closes #235.

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