In the Philippines, Senior Citizens (SC) and Persons With Disabilities (PWD)
are legally entitled to statutory discounts (typically 20% or 5%) and VAT
exemptions on eligible goods and services. Properly handling these
transactions requires removing the standard VAT, recalculating the base
price, and tracking the discount separately on the invoice.

This commit introduces a "Discount Privileges" feature in a new
`l10n_ph_invoice` module, applied directly on customer invoices and credit
notes, so that companies not needing it (e.g. Odoo PH itself) don't have to
deal with it.

Key changes:

* Privilege Model: `l10n_ph.discount.privilege` pairs a discount amount with
  its type (SC/PWD/special), the fiscal position mapping the line taxes to
  the SC/PWD VAT-exempt tax, the target account for the discount journal
  items, and the optional product categories it applies to. Names are
  unique per company and discount amounts are validated between 0 and 100%.
* Shared Mixin: `l10n_ph.discount.privilege.line.mixin` stores the privilege
  and the original price-unit/tax/discount values on any document line and
  computes the regular and special discount amounts, with model-specific
  hooks to derive the gross amounts.
* Invoice Line: `account.move.line` overrides
  `_compute_discount_allocation_needed` to route the discount journal items
  of privileged lines to the privilege account, and provides helpers to
  adjust the price unit and taxes from the privilege's fiscal position
  (restoring the original values when the privilege is removed).
* Application Wizard: `l10n_ph.discount.privilege.wizard` offers per-line
  discount previews, scope filtering (all lines / categories / individual
  products) and apply / remove-all / single-line-remove actions on draft
  customer invoices and credit notes.
* Localization Data (`l10n_ph`): added the "0% EXEMPT FOR SC/PWD USE" tax and
  the "Discount Privileges" fiscal position mapping the standard 12% VAT to
  it, renumbered the chart-of-accounts tax sequences for a logical ordering.
  Default privileges (20% SC, 20% PWD, 5% SC) are created when installing
  the Philippine chart of accounts with `l10n_ph_invoice` installed.
* Configuration: the `l10n_ph` settings now offer a "Discount Privileges"
  toggle that installs the `l10n_ph_invoice` module directly.
* Tests: coverage for the model, wizard flows, fiscal position mappings, and
  discount computations.

Task-6032306

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