[16.0][ADD] sale_invoice_partial_lines

[eantones]

Summary

New module sale_invoice_partial_lines that lets the user pick which sale order lines go into the next regular invoice via a per-line Invoice next checkbox persisted in the database.

• Field: to_invoice_next boolean on sale.order.line (DB-persisted, copy=False). The selection survives pagination, refreshes and browser restarts — important on quotations with hundreds of lines.
• UX: optional column on the embedded order-line tree (visible from confirmed state onwards), footer counter on the sale order form summarising the marked subset (X lines · subtotal Y €), and a search filter on the standalone order-line list view.
• Wizard: standard sale.advance.payment.inv extended with an only_selected_lines checkbox, visible only when Regular invoice is selected. When checked, _get_invoiceable_lines keeps only marked product lines plus their wrapping sections, notes and prior down payments; raises UserError if no line is marked. The flag is reset on every invoiced line right after the invoice is created.
• Tests: 6 cases covering happy path, no-marked-lines validation, post-invoice reset, section pruning, default path parity, and counter computed fields.
• Translations: ca.po and es.po for the new strings only.

Test plan

• Module installs cleanly (-i sale_invoice_partial_lines)
• Tests pass (--test-tags=/sale_invoice_partial_lines, 6/6)
• pre-commit run -a clean
• Visual smoke test on test environment (Carles)

[codecov]

Codecov Report

❌ Patch coverage is 25.88235% with 126 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.79%. Comparing base (20d034b) to head (f11e02c).
⚠️ Report is 1 commits behind head on 16.0.

Files with missing lines	Patch %	Lines
...ial_lines/tests/test_sale_invoice_partial_lines.py	19.04%	85 Missing ⚠️
sale_invoice_partial_lines/models/sale_order.py	21.62%	29 Missing ⚠️
..._partial_lines/wizards/sale_advance_payment_inv.py	33.33%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             16.0     #897      +/-   ##
==========================================
- Coverage   44.28%   43.79%   -0.49%     
==========================================
  Files         317      325       +8     
  Lines        6294     6464     +170     
  Branches     1000     1011      +11     
==========================================
+ Hits         2787     2831      +44     
- Misses       3486     3612     +126     
  Partials       21       21              

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