Expand white-balance support when chromatic adaptation is active

[cgasgarth]

Goal

Remove or narrow the remaining runtime white-balance restrictions that apply when chromatic adaptation is active.

Current limitation

White-balance settings are catalogued, but some writes are still rejected at execution time under active chromatic adaptation.

Current behavior

• numeric WB writes can be blocked
• disabling WB can be blocked
• choice writes are limited in this mode

Starting point

• darktable/src/common/agent_execute.c:279
• darktable/src/common/agent_execute.c:291
• darktable/src/common/agent_execute.c:311

Notes

• Keep the “no corrupt image state” guarantee.
• If full support is not possible, the issue should still aim to reduce over-blocking and make remaining constraints explicit and deterministic.