Add colorKey pipeline op for nodata fill transparency

[pjeweb]

Summary

• Add a colorKey raster pipeline op that turns configured #RRGGBB fill colors transparent, so WMS/aerial nodata can be punched out before merge.
• Default flood starts from the tile edge and already-transparent pixels, so isolated interior matches (roofs, shadows) stay put. fuzz is a per-channel tolerance; fromEdges: false keys every matching pixel.
• Document the op and cover parse, flood, fuzz, cache-hit, and processor wiring with tests.

Test plan

• composer test
• Apply colorKey to a known aerial tile with black nodata and confirm the fill becomes transparent while interior dark roofs remain
• Confirm fromEdges: false also removes isolated interior matches