This commit replaces the percentage-based tax structure and the flat chart of accounts with the ones required by the Finance Act 2026, and removes the outdated tax report.

Before: the 16 tax templates were grouped by rate alone (`VAT at 2%`, `VAT at 4.5%`, ...), which says nothing about the law a tax is levied under. The chart had no hierarchy beyond `active=False` range accounts (`100100-100199`), so nothing rolled up and a section-wise Trial Balance was impossible. `tr_form` no longer matched the NBR reporting format.

Now: taxes sit on the three statutory pillars, Value Added Tax (VAT and Supplementary Duty Act, 2012), Tax Deducted at Source (Withholding Tax Rules, 2026) and VAT Deducted at Source (VAT and Supplementary Duty Rules, 2016), with VDS and TDS flagged as withholding so they are held back until payment rather than deducted on the invoice. The chart is a real `parent_id` hierarchy, `1 Assets` -> `11 Current Assets` -> `111 Cash & Cash Equivalents` -> `111400 Bank Suspense Account`, so accounts roll up by section. `bank_account_code_prefix` moves to `1111` and `_get_account_parent_xmlid` nests the auto-created journal accounts under `111` instead of leaving them outside the tree. The company defaults follow the new chart: VAT 15% on sales and purchases, fiscal year ending 30 June, and the inventory, exchange difference, bank and deferred accounts.

The obsolete tax report is dropped along with its fiscal positions, which mapped nothing once the rate-based taxes were gone.

task-6219535
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For Belgian payroll, employee versions are now considered concurrent if their DIMONA category and worker code match, instead of only contract dates.
So that change the logic in the payslip for where if you select specefic version, the payslip will also include the concurrent versions with the same worker code and DIMONA category.
And if all the versions are concurrent in the payslip period, you will not have an option to select specific version and the payslip will automatically include all the concurrent versions.

For Payrun: now selects employees instead of versions
A payslip is generated for every employee-version combination within the payrun period
If an employee has multiple versions in the payrun period, a payslip is created for each version

The previous tests did not account for the newly added concurrency. In some tests, so that calculate all the concurrent versions in the same payslip and merging two versions into a single payslip hit the contract's wage limit. Since the total payment was capped regardless of the combined payslip amounts, those tests nothing now . To resolve this, I changed the version dates to ensure each version is tested separately. 
Additionally, I removed test_bik_first_payslip_unpaid. That specific scenario no longer exists now that the versions run concurrently.

Task Id: 6126845