Copy target coordinates onto regridded result cube (#6844)

[gaoflow]

🚀 Pull Request

Description

Closes #6844.

After regridding, the horizontal (X/Y) coordinates on the result cube were the same objects as the coordinates on the target grid cube, so mutating one mutated the other:

result = src_cube.regrid(grid_cube, AreaWeighted())
result.coord("longitude") is grid_cube.coord("longitude")   # True!
result.coord("longitude").points += 100
grid_cube.coord("longitude").points                         # also changed!

Cause

iris.analysis._regrid._create_cube builds the result cube and attaches the target's coordinates. Its docstring already states:

Horizontal coordinates are copied from the target cube.

…and the source coordinates it carries across are indeed copied (coord.copy()). But the target coordinates were added by reference:

for tgt_coord, dim in zip(tgt_coords, (grid_dim_x, grid_dim_y)):
    ...
    result.add_dim_coord(tgt_coord, dim)   # same object as the target's

This affects all regridders that share _create_cube. (The curvilinear caller happened to pre-copy its coords, but AreaWeighted and the rectilinear RectilinearRegridder did not — and a regridder also reuses a single target snapshot across calls, so the aliasing could leak between successive results too.)

Fix

Copy each target coordinate inside _create_cube before adding it, matching both the docstring and the existing treatment of source coordinates. This centralises the behaviour for every regridder. The pre-existing .copy() in the curvilinear caller becomes harmlessly redundant.

Verification

• New test Test::test_result_coords_independent_of_target — asserts the result's lat/lon equal the target's but are distinct objects, and that mutating the result leaves the target unchanged. It fails on main and passes here.
• Manually confirmed for AreaWeighted, Linear and Nearest: result coordinates carry the correct points/bounds yet are independent of the target.
• tests/unit/analysis/regrid/ and tests/unit/analysis/area_weighted/ suites pass (81 tests; the 4 Test__derived_coord errors are pre-existing and unrelated — they require optional regridding dependencies not installed locally).
• ruff check / ruff format clean.