fix(eval): truncate DDB period to integer, matching Excel behavior

[TyrGo]

The DDB function applied an incorrect weighted-average blending for fractional period values. Excel (and the sibling DB function in this codebase) truncates the period argument to an integer before calculating depreciation.

Changes:

• Truncate period to integer before the depreciation loop, consistent with DB and Excel behavior.
• Move the period bounds check after truncation so that fractional values like 5.9 (which truncate to 5) are accepted when life=5, while sub-unit values like 0.5 (which truncate to 0) correctly return #NUM!.
• Remove the incorrect fractional-period blending block and its TODO.
• Update docstring to document truncation behavior and add FAQ entry.
• Add inline example for fractional period truncation.
• Add 10 JSON formula tests covering: second period, final period with salvage floor, fractional periods (0.5, 1.9, 2.5, 5.9), custom factor, and error cases (period=0, period>life, negative period).

[Copilot]

Pull request overview

This PR fixes the DDB depreciation function to truncate fractional period inputs to an integer before validation and calculation, matching Excel behavior and aligning with how the existing DB implementation handles period iteration.

Changes:

• Truncate period to an integer prior to bounds checking and the depreciation loop.
• Remove the prior fractional-period weighted blending logic.
• Add JSON formula tests covering integer, fractional, and error-case periods, plus a custom factor case.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
tests/formula_tests/financial.json	Adds coverage for DDB across multiple periods, fractional truncation cases, and #NUM! error scenarios.
crates/formualizer-eval/src/builtins/financial/depreciation.rs	Updates DDB implementation to use integer-truncated period, removes fractional blending, and refreshes docs/examples/FAQ accordingly.

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