[14.0][FIX] product_multi_company: allow editing company_ids on product variant

[Mert-coderoid]

The variant's company_ids is a stored compute field derived from the template. Without an inverse, any write (e.g. from the variant form) gets overwritten by the next compute pass, so attempts to assign companies on a product.product are silently lost.

This is observable in setups where end users navigate to product.product (variant) views instead of product.template — the field renders, the user picks companies, hits Save, and the value disappears on reload.

Fix

Add an inverse that propagates the change to the template, using sudo() so users with variant write access can still trigger the propagation (the template-level rule may be stricter).

The propagation naturally syncs all variants of the same template, which matches the module's existing model (companies live on the template; variants follow).

Tested

• Write [c1, c2] on a variant → both variant and template end up [c1, c2]
• Subset write ([c1] while template is [c1, c2]) → template shrinks to [c1] (expected)
• Empty write → both variant and template empty
• Re-write same value → no loop (~8ms)
• Multi-record write across variants of different templates → each template updated correctly
• Write by a user with no template-write rule → succeeds via sudo()