[16.0] [FIX] stock_available_immediately: kit immediately_usable_qty computed from components - #73

For products with a phantom BoM (kits), the formula
immediately_usable_qty = virtual_available - incoming_qty applied to the
aggregated kit fields produces inconsistent results because each aggregated
kit field (virtual_available, incoming_qty, ...) is computed by Odoo MRP
as an independent min over the components and may be limited by different
components. Subtracting two such mins is not equivalent to
qty_available - outgoing_qty for kits, even though that algebraic
cancellation does hold for plain products.

Example

Kit with two components, need=2 each:

Component A: qty=8,    in=133, out=14   →  virtual=127,  immediately=-6
Component B: qty=4258, in=60,  out=168  →  virtual=4150, immediately=4090

Forecasted (kit) = min(127/2, 4150/2)  = 63    ← limited by A
Incoming   (kit) = min(133/2, 60/2)    = 30    ← limited by B
immediately_usable_qty (current bug) = 63 - 30 = 33   ← inflated, not physical
immediately_usable_qty (expected)    = min(-6/2, 4090/2) = -3

Fix

When the mrp module is installed, recompute immediately_usable_qty for
products with a phantom BoM directly from the components (whose individual
values do hold the correct cancellation) and aggregate them with
min(component / qty_per_kit). The other kit fields (qty_available,
virtual_available, incoming_qty, outgoing_qty, free_qty) are
intentionally left untouched and keep the standard Odoo MRP aggregation.

If mrp is not installed, behaviour is unchanged.

Tests

Added covering:

• Unbalanced components reproducing the bug → expected negative result.
• Balanced components → no change vs. previous behaviour.
• All-positive components → standard healthy case.
• Oversold components → propagation to the kit value.
• Plain products without BoM → not affected (regression).
• Consumable components → skipped in the ratio, only storable components limit.