Fix --permissive with balance assignments (#2005)

[jwiegley]

Summary

• When a posting uses balance assignment syntax with no explicit amount (e.g., Assets:Checking =$12345) and the computed amount causes the transaction not to balance, --permissive now accepts the transaction by reverting to auto-balancing
• The fix is in finalize() (Phase 8 of xact.cc), not in the parser — this avoids the regression from PR fix: respect --permissive for balance assignments with elided amounts #2710 which broke issue Permissive mode now breaks balance assertions #2944 by skipping assignment computation entirely
• Balance assignments that DO balance (like Permissive mode now breaks balance assertions #2944) are unaffected because they never reach Phase 8's error path

Approach

The previous fix (PR #2710, reverted in PR #2964) modified the parser to skip balance assignment computation under --permissive. This broke legitimate balance assignments (#2944) where the computed amount balances correctly.

This fix takes a different approach: the parser always computes the assignment amount. In finalize(), after detecting an imbalance but before throwing, we check if --permissive is active and a POST_CALCULATED balance-assignment posting exists. If so, we adjust that posting's amount to absorb the imbalance, effectively auto-balancing the transaction.

Test plan

• test/regress/2005.test passes — --permissive accepts the unbalanced assignment
• test/regress/2005.test also verifies the error is still raised WITHOUT --permissive
• test/regress/2944.test passes — balance assignments that balance are unaffected
• Full test suite: 4096/4096 tests pass
• nix build . passes

Fixes #2005

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