fix(model): floor negative Unix timestamps to seconds

[jakezwang]

model.Time(-1).Unix() returns 0, while the same instant converted with Time().Unix() returns -1. Integer division truncates negative fractional seconds toward zero.

Reuse the existing time.Time conversion so Unix seconds round down consistently. Tests cover both sides of the epoch, exact seconds, and the earliest/latest representable millisecond timestamps.

Validation: the four negative fractional cases fail before the fix. make test passes on Go 1.25.13 and 1.26.6, including the full race suite and pinned golangci-lint. License/style and asset checks pass. Both modules declare Go 1.25.0; the shell version-check script requires readarray, unavailable in the local macOS Bash.

@roidelapluie