readGsdEffectiveModelOverrides resolved ~/.gsd/defaults.json via os.homedir()
with no seam, so a test asserting project-only overrides could not isolate the
global file. Add an optional { homedir } option (defaults to os.homedir()) to
readGsdGlobalModelOverrides and readGsdEffectiveModelOverrides — the same
dependency-injection shape the sibling warnIfStaleBake already uses. Backward-
compatible: existing callers pass no option and behave identically.
Closes#2152